ASYMPTOTIC BEHAVIOR OF COMPARABLE SKEW-PRODUCTSEMIFLOWS WITH APPLICATIONS
FENG CAO, MATS GYLLENBERG, AND YI WANG
Abstract. The (almost) 1-cover lifting property of omega-limit sets is es-
tablished for nonmonotone skew-product semiflows, which are comparable to
uniformly stable eventually strongly monotone skew-product semiflows. These
results are then applied to study the asymptotic behavior of solutions to the
nonmonotone comparable systems of ODEs, reaction-diffusion systems, differ-
ential systems with time delays and semilinear parabolic equations.
1. Introduction
Monotone dynamical systems have been widely studied because these systems
provide a unified relevant mathematical framework for the qualitative analysis of
many important equations, including second-order parabolic equations and various
classes of systems of ordinary, parabolic and functional differential equations. We
refer to [18, 43] for a comprehensive survey on development of this theory.
The path-breaking work by Hirsch [17] showed that trajectories in strongly
monotone systems have a strong tendency not to be chaotic, i.e., almost all of their
omega-limit sets consist of equilibria. For smooth strongly monotone systems, the
forward orbits are generically convergent to equilibria in the continuous-time case or
to cycles in the discrete-time case (see, e.g. [37, 38, 43]). Recently, non-periodic and
non-autonomous equations have been attracting more attention. A unified frame-
work to study nonautonomous equations is based on the so-called skew-product
semiflows (see [39, 41], etc). In contrast to the autonomous and periodic cases, the
generic convergence cannot hold in strongly monotone skew-product semiflows (See
[41]), even for quasi-periodic or almost periodic cases.
During the past 20 years, many researchers in this field have tried to impose
additional conditions to obtain more useful information of the structure of the
2000 Mathematics Subject Classification. 37B55, 37L15, 34K14, 35B15, 35K57.
Y.W. is partially supported by the Finnish Center of Excellence in Analysis and Dynamics,
and NSF of China No.10971208.
1
2 FENG CAO, MATS GYLLENBERG, AND YI WANG
limit sets of the orbits in monotone systems. One of the popular approaches is to
provide reasonable assumptions to guarantee global convergence of the orbits. Such
assumptions include subhomogeneity ([21, 33, 34, 44, 47, 50, 53]); minimal equilibria
([14, 51]); a first integral with positive gradient ([5, 6, 22, 29, 42, 48]), phase-
translation invariance ([4, 25]). It is worth pointing out that all these additional
assumptions make the systems “orbitally stable” in a certain sense (see [1, 2, 24]) .
However, it is well known that a large amount of important evolution equa-
tions do not generate monotone systems. For example, many population models
with non-cooperative growth functions, and reaction-diffusion systems with non-
quasimonotone reaction terms are among such equations. In order to study proper-
ties of the solutions of such non-monotone evolution equations, an effective approach
is to exhibit and utilize certain comparison techniques. Historically, the motivation
for this approach was to obtain upper (maximal) and lower (minimal) solutions to a
given evolution equation (see the pioneering works in [10, 19] for partial differential
equations, and [26, 31] for ordinary differential equations).
A remarkable comparison technique was derived by Conway and Smoller [9]
for nonmonotone reaction-diffusion systems which admit an invariant rectangle.
This then triggered the successful investigation of global dynamical behavior of
the differential equations with spatial structure arising in mathematical ecology
and population biology (see, e.g., [7, 8, 46], etc). As pointed out in [45, Section
4], it turns out that this comparison technique involves monotone systems in a
very natural way: the original nonmonotone system is comparable with respect to
certain monotone systems.
Motivated by such insight, given a nonmonotone system which is comparable
to some monotone system, one would want to know whether such a nonmonotone
system inherits certain asymptotic behavior from its monotone “partner”. The
answer is generally negative even in autonomous cases unless we impose additional
conditions on the associated monotone systems. To the best of our knowledge,
there are only a few works on the related topics, such as contracting rectangle
techniques in systems of reaction-diffusion equations ([7, 46]) and delayed-equations
([28, 43]), pseudo monotone approaches in functional differential equations ([11, 52])
and sandwich methods in integrodifference equations ([20]), etc. Recently, Jiang
[23] discovered the global convergence of the comparable nonmonotone discrete-
time or continuous-time system provided that all the equilibria of its monotone
partner form into a totally ordered curve in the phase space.
COMPARABLE SKEW-PRODUCT SEMIFLOWS 3
The purpose of our current paper is to study the global dynamics of compara-
ble nonmonotone skew-product semiflows (see Definition 4.1) with respect to some
eventually strongly monotone partner. Since even the strongly monotone skew-
product semiflows can possess very complicated chaotic attractors (see [41]), we
henceforth restrict our attention to the monotone partners which are “uniformly
stable”. According to [24], such monotone skew-product semiflows are globally
tamed-behaved: every precompact trajectory is asymptotic to a copy (also called
1-cover) of the base flow. As a starting point in this paper, for the uniformly sta-
ble eventually strongly monotone skew-product semiflows, we will first investigate
the topological structure of the set of the union of all 1-covers (see Theorem 3.1).
Roughly speaking, this set is a 1-dimensional continuous subbundle on the base,
while each fibre of such bundle is totally ordered and homeomorphic to a closed
interval in R. Moreover, all the fibers share a common “bounded-or-unbounded”
property uniformly for all the base point (see Remark 3.2). Armed with such key
tools, we are able to show the (almost) 1-covering property of omega-limit sets of
comparable nonmonotone skew-product semiflows, whose partner systems are even-
tually strongly monotone and uniformly stable (see Theorem 4.3 in detail). Then
these results are applied to study the asymptotic almost periodicity of solutions to
almost periodic reaction-diffusion systems, ODE systems, differential systems with
time delays and time-recurrent parabolic equations.
This paper is organized as follows. In section 2, we present some basic con-
cepts and preliminary results in the theory of skew-product semiflows and almost
periodic functions which will be important to our proofs. Section 3 is devoted to
the establishment of the topological structure of the union of all 1-covers for even-
tually strongly monotone and uniformly stable skew-product semiflows (Theorem
3.1). In Section 4, we prove the lifting property of the ω-limit sets of compara-
ble nonmonotone skew-product semiflows (Theorem 4.3). In section 5, we apply
our abstract theorems in Sections 3 and 4 to obtain the asymptotic almost peri-
odicity of solutions to comparable systems as non-cooperative ordinary differential
systems (Section 5.1), non-quasimonotone reaction-diffusion systems (Section 5.2)
and time-delayed systems (Section 5.3), and comparable almost periodic parabolic
equations (Section 5.4).
4 FENG CAO, MATS GYLLENBERG, AND YI WANG
2. Preliminaries
In this section, we summarize some preliminary material to be used in later
sections. First, we summarize some lifting properties of compact dynamical sys-
tems. We then collect definitions and basic facts concerning eventually strongly
monotone skew-product semiflows. Finally, we give a brief review about uniformly
almost periodic functions.
Let Ω be a compact metric space with metric dΩ, and σ : Ω × R → Ω be a
continuous flow on Ω, denoted by (Ω, σ) or (Ω,R). As has become customary,
we denote the value of σ at (ω, t) alternatively by σt(ω) or ω · t. By definition,
σ0(ω) = ω and σt+s(ω) = σt(σs(ω)) for all t, s ∈ R and ω ∈ Ω. A subset S ⊂ Ω is
invariant if σt(S) = S for every t ∈ R. A non-empty compact invariant set S ⊂ Ω
is called minimal if it contains no non-empty, proper and invariant subset. We
say that the continuous flow (Ω,R) is minimal if Ω itself is a minimal set; distal if
inft∈R
dΩ(σt(ω1), σt(ω2)) > 0 whenever ω1, ω2 ∈ Ω and ω1 6= ω2. Let (Z,R) be another
continuous flow. A continuous map p : Z → Ω is called a flow homomorphism if
p(z · t) = p(z) · t for all z ∈ Z and t ∈ R. A flow homomorphism which is onto
is called a flow epimorphism and a one-to-one flow epimorphism is referred as a
flow isomorphism. We note that a homomorphism of minimal flows is already an
epimorphism.
Throughout this paper, we always assume that the flow (Ω,R) is minimal and
distal.
We say that a Banach space (V, ‖·‖) is strongly ordered if it contains a closed
convex cone, that is, a non-empty closed subset V+ ⊂ V satisfying V+ + V+ ⊂ V+,
αV+ ⊂ V+ for all α ≥ 0, and V+ ∩ (−V+) = 0 with non-empty interior IntV+ 6= ∅(also call that V+ is solid). The cone V+ induces a strong ordering on V via
x1 ≤ x2 if x2 − x1 ∈ V+. We write x1 < x2 if x2 − x1 ∈ V+ \ 0, and x1 ¿ x2
if x2 − x1 ∈ IntV+. Given x1, x2 ∈ V , the set [x1, x2] = x ∈ V : x1 ≤ x ≤ x2 is
called a closed order interval in V , and we write (x1, x2) = x ∈ V : x1 < x < x2.Let x ∈ V and a subset U ⊂ V . We write x <r U if x <r u for all u ∈ U , where <r
represents ≤ or <. x >r U is similarly defined.
A subset U of V is said to be order convex if for any a, b ∈ U with a < b,
the segment a + s(b − a) : s ∈ [0, 1] is contained in U . And U is called lower-
bounded (resp. upper-bounded) if there exists an element a ∈ V such that a ≤ U
(resp. a ≥ U). Such an a is said to be a lower bound (resp. upper bound) for U .
COMPARABLE SKEW-PRODUCT SEMIFLOWS 5
U is called lower-unbounded (resp. upper-unbounded), if it is not lower-bounded
(resp. upper-bounded); U is called bounded if U is both lower-bounded and upper-
bounded, otherwise it is unbounded. Moreover, U is totally unbounded if it is both
lower-unbounded and upper-unbounded.
A lower bound a0 is said to be the greatest lower bound (g.l.b.), if any other lower
bound a satisfies a ≤ a0. Similarly, we can define the least upper bound (l.u.b.).
Let X = [a, b]V with a ¿ b (a, b ∈ V ) or X = V+, or furthermore, X be a closed
order convex subset of V . Our first standing hypothesis is
(H1) Every compact subset in X has both a greatest lower bound and a least
upper bound.
Let R+ = t ∈ R : t ≥ 0. We consider a continuous skew-product semiflow
Π : Ω×X × R+ → Ω×X defined by
(2.1) Πt(ω, x) = (ω · t, u(t, ω, x)) , ∀(t, ω, x) ∈ R+ × Ω×X,
satisfying (1) Π0 = Id; (2) the cocycle property : u(t+s, ω, x) = u (s, ω · t, u(t, ω, x)),
for each (ω, x) ∈ Ω×X and s, t ∈ R+. Our next standing hypothesis is
(H2) For every (ω, x) ∈ Ω×X, there is a t0 = t0(ω, x) > 0 such that Π(t, ω, x) :
t ≥ t0 is precompact.
A subset A ⊂ Ω×X is positively invariant if Πt(A) ⊂ A for all t ∈ R+; and totally
invariant if Πt(A) = A for all t ∈ R+. The forward orbit of any (ω, x) ∈ Ω×X is
defined by O+(ω, x) = Πt(ω, x) : t ≥ 0, and the omega-limit set of (ω, x) is defined
byO(ω, x) = (ω, x) ∈ Ω×X : Πtn(ω, x) → (ω, x)(n →∞) for some sequence tn →
∞. By (H2), every omega-limit set O(ω, x) is a nonempty, compact and totally
invariant subset in Ω×X for Πt.
Let P : Ω ×X → Ω be the natural projection. A compact positively invariant
set K ⊂ Ω×X is called an almost 1-cover (or almost automorphic extension) of Ω
if there exists ω0 ∈ Ω such that P−1(ω0) ∩K consists of a unique element. And,
K is a 1-cover of the base flow if P−1(ω) ∩K contains a unique element for every
ω ∈ Ω. In this case, we denote the unique element of P−1(ω) ∩K by (ω, c(ω)) and
write K = (ω, c(ω)) : ω ∈ Ω, where c : Ω → X is continuous with
Πt(ω, c(ω)) = (ω · t, c(ω · t)), ∀t ≥ 0,
6 FENG CAO, MATS GYLLENBERG, AND YI WANG
and hence, K ∩ P−1(ω) = (ω, c(ω)) for every ω ∈ Ω. For the sake of brevity,
we hereafter also write c(·) as a 1-cover of Ω and K ∩ P−1(ω) = (ω, c(ω)) in the
context without any confusion.
Next, we introduce some definitions concerning compactness and stability of the
skew-product semiflow Πt.
Definition 2.1. (1) (Fiber compactness) Πt is fiber-compact if, there exists a t0 > 0
such that for any ω ∈ Ω and bounded subset B ⊂ X, Πt(ω, B) has compact closure
in P−1(ω · t) for every t > t0.
(2) (Uniform stability) A forward orbit O+(ω0, x0) of Πt is said to be uniformly
stable if for every ε > 0 there is a δ = δ(ε) > 0, called the modulus of uniform
stability, such that, for every x ∈ X, if s ≥ 0 and ‖u(s, ω0, x0)− u(s, ω0, x)‖ ≤ δ(ε)
then
‖u(t + s, ω0, x0)− u(t + s, ω0, x)‖ < ε for each t ≥ 0.
For skew-product semiflows, we always use the order relation on each fiber
P−1(ω). We write (ω, x1) ≤ω (<ω,¿ω) (ω, x2) if x1 ≤ x2 (x1 < x2, x1 ¿ x2).
Without any confusion, we will drop the subscript “ω”. One can also define similar
definitions and notations in P−1(ω) as in X, such as order-intervals, the greatest
lower bound, the least upper bound, etc.
Definition 2.2. The skew-product semiflow Π is monotone if
Πt(ω, x1) ≤ Πt(ω, x2)
whenever (ω, x1) ≤ (ω, x2) and t ≥ 0. Moreover, Π is eventually strongly monotone
if it is monotone and there exists a t0 > 0 such that
Πt(ω, x1) ¿ Πt(ω, x2) whenever (ω, x1) < (ω, x2) and t ≥ t0.
Now we present the third Standing Hypothesis:
(H3) The skew-product semiflow Πt : Ω × X → Ω × X is eventually strongly
monotone, and every forward orbit of Πt is uniformly stable.
The following result is adopted from [24] and will play an important role in our
forthcoming sections.
Lemma 2.3. Assume that (H1)-(H3) hold. Then for any (ω0, x0) ∈ Ω × X, its
omega-limit set O(ω0, x0) is a 1-cover of the base flow (Ω,R).
COMPARABLE SKEW-PRODUCT SEMIFLOWS 7
Proof. See Theorem 4.1 in [24]. ¤
We finish this section with the definitions of almost periodic functions and flows.
A function f ∈ C(R,Rn) is almost periodic if, for any ε > 0, the set T (ε) := τ :
|f(t + τ)− f(t)| < ε, ∀t ∈ R is relatively dense in R. Let D ⊆ Rm be a subset of
Rm. A continuous function f : R×D → Rn; (t, u) 7→ f(t, u), is said to be admissible
if f(t, u) is bounded and uniformly continuous on R × K for any compact subset
K ⊂ D. A function f ∈ C(R×D,Rn)(D ⊂ Rm) is uniformly almost periodic in t,
if f is both admissible and almost periodic in t ∈ R.
Let f ∈ C(R ×D,Rn)(D ⊂ Rm) be admissible. Then H(f) = clf · τ : τ ∈ Ris called the hull of f , where f · τ(t, ·) = f(t + τ, ·) and the closure is taken under
the compact open topology. Moreover, H(f) is compact and metrizable under the
compact open topology. The time translation g · t of g ∈ H(f) induces a natural
flow on H(f). H(f) is always minimal and distal whenever f is a uniformly almost
periodic function in t (see, e.g., [39] or [41]).
Let f ∈ C(R× Rn,Rn) be uniformly almost periodic, and
(2.2) f(t, x) ∼∑
λ∈Raλ(x)eiλt
be a Fourier series of f (see [49, 41] for the definition and the existence of Fourier
series). Then S = λ : aλ(x) 6≡ 0 is called the Fourier spectrum of f associated to
the Fourier series (2.2), andM(f) = the smallest additive subgroup of R containing
S(f) is called the frequency module of f . Moreover, M(f) is a countable subset of
R. Let f, g ∈ C(R× Rn,Rn) be two uniformly almost periodic functions in t. The
module containment M(f) ⊂ M(g) if and only if there exists a flow epimorphism
from H(g) to H(f) (see, [12] or [41, Section 1.3.4]). In particular, M(f) = M(g)
if and only if the flow (H(g),R) is isomorphic to the flow (H(f),R).
3. Topological structure of the union of all 1-covers for Πt
Throughout this paper, we always assume that the skew-product semiflow Πt
satisfying (H1)-(H3). Therefore, by Lemma 2.3, the omega-limit set O(ω, x) of
every (ω, x) ∈ Ω×X is a 1-cover (of Ω) for Πt.
Let C(Π) = c(·) : c(·) is a 1-cover for Πt. For a(·), b(·) ∈ C(Π), we write
a(·) <r b(·) if (ω, a(ω)) <r (ω, b(ω)) for every ω ∈ Ω. Here the order “<r”
means “≤”, “<” or “¿”. If a(·) ≤ b(·), we define the 1-cover closed order-interval
8 FENG CAO, MATS GYLLENBERG, AND YI WANG
[a(·), b(·)]C(Π) = w(·) ∈ C(Π) : a(·) ≤ w(·) ≤ b(·), and the 1-cover open order-
interval (a(·), b(·))C(Π) = w(·) ∈ C(Π) : a(·) ¿ w(·) ¿ b(·). Due to the eventu-
ally strong monotonicity, one can also write (a(·), b(·))C(Π) = w(·) ∈ C(Π) : a(·) <
w(·) < b(·).Consider the union
A =⋃
K is a 1-cover for Πt
K
of all 1-covers (of Ω) for Πt in Ω×X. Then, A is an invariant subset of Ω×X. For
each ω ∈ Ω, we write A(ω) = P−1(ω) ∩A. It is also easy to see that
A(ω) = (ω, u)|u = a(ω) for some a(·) ∈ C(Π).
Now we are in a position to describe the topological structure of A, which is our
main result in this section.
Theorem 3.1 (Topological structure of union of 1-covers). Assume that (H1)-(H3)
hold and Πt is fiber-compact. Then, for each ω ∈ Ω, A(ω) is totally ordered and
closed in P−1(ω). Moreover, there is a continuous bijective mapping h : Ω×I → A,
where the interval I = 0, [0, 1], [0,+∞), (−∞, 0] or (−∞,+∞), satisfying
(i) For each α ∈ I, h(·, α) = (·, a(·)) for some a(·) ∈ C(Π);
(ii) For each ω ∈ Ω, h(ω, I) = A(ω). In addition, if I is nontrivial (i.e., I 6= 0),then h is strictly order-preserving with respect to α ∈ I, i.e.,
h(ω, α1) ¿ h(ω, α2)
for any ω ∈ Ω and any α1, α2 ∈ I with α1 < α2;
(iii) I = 0 or [0, 1] corresponds to the case that A is compact, in which A(ω)
is bounded for all ω ∈ Ω;
(iv) I = [0,+∞) (resp.(−∞, 0]) corresponds to the case that A(ω) is unbounded
but lower-bounded (resp. upper-bounded), for all ω ∈ Ω;
(v) I = (−∞,+∞) corresponds to the case that A(ω) is totally unbounded, for
all ω ∈ Ω.
Remark 3.2. Because of items (iii)-(v) of above Theorem, we say that the fibers
A(ω) share the common “bounded-or-unbounded property” uniformly for all the base
point ω ∈ Ω.
In order to prove Theorem 3.1, we need a series of important lemmas.
COMPARABLE SKEW-PRODUCT SEMIFLOWS 9
Lemma 3.3. Assume that (H1)-(H3) hold. Let p(·), q(·) ∈ C(Π) with p(·) < q(·).Then there exists some r(·) ∈ C(Π) such that p(·) < r(·) < q(·). Moreover, one has
p(·) ¿ r(·) ¿ q(·).
Proof. By the eventually strong monotonicity of Πt, one may assume that p(·) ¿q(·). Suppose that there exists no 1-cover of Ω in (p(·), q(·))C(Π). Then we assert
that the following Order-Interval Dichotomy holds: For each ω ∈ Ω, either
(i) O(ω, x) = p(·), ∀x ∈ (p(ω), q(ω)); or
(ii) O(ω, x) = q(·), ∀x ∈ (p(ω), q(ω)).
Before giving the proof of the Order-Interval Dichotomy, we note that such
Order-Interval Dichotomy implies that neither the orbit Πt(ω, q(ω)) nor the orbit
Πt(ω, p(ω)) is uniformly stable, which contradicts our fundamental hypothesis (H3).
Thus, there exists some r(·) ∈ C(Π) such that p(·) < r(·) < q(·). Again by the
eventually strong monotonicity of Πt, we obtain that p(·) ¿ r(·) ¿ q(·).So, it remains to prove the Order-Interval Dichotomy. To end this, suppose on
the contrary that one can find an ω0 ∈ Ω such that neither (i) nor (ii) holds. Denote
by L the open line segment with endpoints (ω0, p(ω0)) and (ω0, q(ω0)). Then, by
the choice of ω0 and the monotonicity of Πt, it is easy to see that
neither p(·) nor q(·) can attract the whole L.(3.1)
Recall that there exists no 1-cover of Ω in (p(·), q(·))C(Π). It then follows from
Lemma 2.3 that either
O(ω0, x) ∩ P−1(ω0) = (ω0, p(ω0))
or
O(ω0, x) ∩ P−1(ω0) = (ω0, q(ω0))
for every (ω0, x) ∈ L. Combining with (3.1), the monotonicity of Πt implies that
there exists (ω0, e) ∈ L such that
O(ω0, x) ∩ P−1(ω0) = (ω0, p(ω0))
for (ω0, x) ∈ L with x < e, and
O(ω0, x) ∩ P−1(ω0) = (ω0, q(ω0))
for (ω0, x) ∈ L with x > e. Without loss of generality, one may also assume that
O(ω0, e) ∩ P−1(ω0) = (ω0, p(ω0)).
10 FENG CAO, MATS GYLLENBERG, AND YI WANG
Let ε0 := ‖q(ω0)− p(ω0)‖ > 0. Since Πt(ω0, e) is uniformly stable, there is a δ0 > 0
such that
(3.2) ‖u(t, ω0, e∗)− u(t, ω0, e)‖ <
ε02
whenever t ≥ 0 and ‖e∗ − e‖ < δ0. Let e∗ > e, (ω0, e∗) ∈ L with ‖e∗ − e‖ < δ0 and
choose tn → +∞ such that ω0 · tn → ω0, it follows from (3.2) that
(3.3) ‖u(tn, ω0, e∗)− u(tn, ω0, e)‖ <
ε02
.
Let n →∞ in (3.3), we have
ε0 = ‖q(ω0)− p(ω0)‖ ≤ ε02
,
a contradiction. Thus, we have proved the Order-Interval Dichotomy, which com-
pletes the proof of the lemma. ¤
Lemma 3.4. Assume that (H1)-(H3) hold and Πt is fiber-compact. Let a(·), b(·) ∈C(Π) with a(·) < b(·). Then, for every ω ∈ Ω, there exists a strictly order-preserving
continuous path
Jω : [0, 1] → A(ω)
with endpoints Jω(0) = (ω, a(ω)) and Jω(1) = (ω, b(ω)).
Proof. Similarly as Lemma 3.3, we again assume that a(·) ¿ b(·). Let
Y = Y ⊂ [a(·), b(·)]C(Π) : Y is a totally ordered subset w.r.t. “≤”.
Then (Y,⊂) is a partially-ordered set. By Zorn’s lemma, we obtain that Y possesses
a maximal element, say Y ∗. Obviously, a(·), b(·) ∈ Y ∗ ⊂ [a(·), b(·)]C(Π).
Now we show that Y ∗ has the following properties:
(i) p(·), q(·) ∈ Y ∗ and p(·) < q(·) implies p(·) ¿ r(·) ¿ q(·) for some r(·) ∈ Y ∗;
(ii) Y ∗(ω) := (ω, w(ω)) : w(·) ∈ Y ∗ is compact for any ω ∈ Ω;
(iii) Y ∗(ω) is connected for any ω ∈ Ω.
Before giving the proof of (i)-(iii), we point out that such properties imply the
existence of the path Jω, due to Proposition Y1 in [36, Page 434, Appendix]. More
precisely, if (i)-(iii) are satisfied, then for every ω ∈ Ω, there exists a homeomor-
phism
Jω : [0, 1] → Y ∗(ω) ⊂ A(ω),
which is order-preserving such that Jω(0) = (ω, a(ω)) and Jω(1) = (ω, b(ω)). So it
remains to prove (i)-(iii). We shall discuss them one by one.
COMPARABLE SKEW-PRODUCT SEMIFLOWS 11
(i) Let p(·), q(·) ∈ Y ∗ with p(·) ¿ q(·). It then follows from Lemma 3.3 that
there exists some c(·) ∈ (p(·), q(·))C(Π). Suppose that (p(·), q(·))C(Π) ∩ Y ∗ = ∅.Then c(·) is order-related to any element in Y ∗, because Y ∗ is totally ordered. As
a consequence, the set Y ′ = Y ∗ ∪ c(·) is a totally ordered set in [a(·), b(·)]C(Π),
which contradicts the maximality of Y ∗. Hence, (p(·), q(·))C(Π) ∩ Y ∗ 6= ∅, that is,
one can find a r(·) ∈ Y ∗ such that p(·) ¿ r(·) ¿ q(·).
(ii) Note that Πt is fiber-compact and Y ∗(ω) ⊂ Πt0(ω ·−t0, [a(ω ·−t0), b(ω ·−t0)]).
Then Y ∗(ω) has compact closure for every ω ∈ Ω. So in order to prove Y ∗(ω) is
compact, it suffices to show that
(3.4) Y ∗(ω) is closed for every ω ∈ Ω.
Indeed, given any ω ∈ Ω and any sequence (ω, xm(ω))∞m=1 ⊂ Y ∗(ω) such that
xm(ω) → c as m → ∞, it is easy to see that c ∈ [a(ω), b(ω)]. Since Πt(ω, c) is
uniformly stable, for every ε > 0 there is a positive integer M = M(ε) ∈ N such
that
(3.5) ‖u(t, ω, xm(ω))− u(t, ω, c)‖ < ε, ∀t ≥ 0, m ≥ M(ε).
Let c∗(·) ∈ C(Π) be the ω-limit set of (ω, c), and choose a sequence tn → +∞ such
that ω · tn → ω as n →∞. Recall that xm(·) ∈ C(Π) for all m = 1, 2, · · · . It then
follows from (3.5) that
‖xm(ω)− c∗(ω)‖ ≤ ε, for all m ≥ M(ε).
Letting m →∞, it yields that
‖c− c∗(ω)‖ ≤ ε.
Note that ε > 0 is arbitrarily chosen. So, one obtains that (ω, c) = (ω, c∗(ω)) ∈A(ω). Recall that Y ∗ is totally ordered. Then for any y(·) ∈ Y ∗, there exists a
subsequence of xm(·)∞m=1, still denoted by xm(·)∞m=1, such that either xm(·) ≤y(·) for all m ∈ N, or xm(·) ≥ y(·) for all m ∈ N. As a consequence, one obtains
that y(·) and c∗(·) is related by “≤”. By arbitrariness of y(·) ∈ Y ∗, c∗(·) is related
to any element in Y ∗. Suppose that c∗(·) /∈ Y ∗. Then Y ′ = Y ∗ ∪ c∗(·) is also
totally ordered. This contradicts the maximality of Y ∗. Accordingly we conclude
that c∗(·) ∈ Y ∗, and hence (ω, c) = (ω, c∗(ω)) ∈ Y ∗(ω), which completes the proof
of (3.4).
12 FENG CAO, MATS GYLLENBERG, AND YI WANG
(iii) Suppose that Y ∗(ω) is not connected. Then one can find a nonempty proper
subset S ( Y ∗(ω), which is both open and closed in Y ∗(ω). Choose some (ω, b) ∈Y ∗(ω) \ S. Since Y ∗(ω) is totally ordered with respect to “¿”, we define the set
S− = (ω, y) ∈ S : (ω, y) ¿ (ω, b) and S+ = (ω, y) ∈ S : (ω, y) À (ω, b). Then
S = S− ∪ S+, and hence, one may assume without loss of generality that S− 6= ∅.Recall that Y ∗(ω) is compact. Then a(ω) , l.u.b.S− exists. Moreover a(ω) ∈ S−,
because S is closed and (ω, b) /∈ S . On the other hand, note that a(ω) ∈ S and S
is also open in Y ∗(ω). Then there exists (ω, c) ∈ S− such that a(ω) ¿ (ω, c), which
contracts the definition of a(ω). Thus, we have proved that Y ∗(ω) is connected.
This completes the proof. ¤
Lemma 3.5. Assume that a(·), b(·) ∈ C(Π) with a(·) ¿ b(·). Given ω0 ∈ Ω, let
J : [0, 1] → A(ω0) be a strictly order-preserving continuous path in A(ω0), with
endpoints J(0) = (ω0, a(ω0)) and J(1) = (ω0, b(ω0)). Then, for any ω ∈ Ω and
x ∈ [a(ω), b(ω)], there exists some τ = τ(ω, x) ∈ [0, 1] such that
O(ω, x) ∩ P−1(ω0) = J(τ).
Proof. For any ω ∈ Ω and x ∈ [a(ω), b(ω)], it follows from Lemma 2.3 that
O(ω, x) = c(·) for some c(·) ∈ C(Π). Let α and β, respectively, be the largest
and the smallest numbers in [0, 1] ⊂ R satisfying
(3.6) J(α) ≤ (ω0, c(ω0)) ≤ J(β).
Thus, it suffices to prove that α = β. Suppose on the contrary that α < β. Then,
either c(ω0) 6= J(α) or c(ω0) 6= J(β). Note that Πt is eventually strongly monotone.
Then by (3.6), one obtains that either J(α) ¿ (ω0, c(ω0)) or (ω0, c(ω0)) ¿ J(β).
But this contradicts the definition of α or β. Thus we conclude that α = β =
τ(ω, x) ∈ [0, 1]. ¤
Now we are ready to prove Theorem 3.1.
Proof of Theorem 3.1. First we claim that any two distinct elements of C(Π) are
related with respect to “¿”. Indeed, take u(·), v(·) ∈ C(Π). For each ω ∈ Ω, we
define
x(ω) = g.l.b.u(ω), v(ω)
and
y(ω) = l.u.b.u(ω), v(ω).
COMPARABLE SKEW-PRODUCT SEMIFLOWS 13
Now fix an ω0 ∈ Ω, let p(·) = O(ω0, x(ω0)) and q(·) = O(ω0, y(ω0)), with p(·), q(·) ∈C(Π), respectively. Using the eventually strong monotonicity of Πt, one obtains
p(·) ≤ u(·) and p(·) ≤ v(·). Similarly, u(·) ≤ q(·) and v(·) ≤ q(·). Accordingly,
u(·), v(·) ∈ [p(·), q(·)]C(Π)
with p(·) ¿ q(·). By virtue of Lemma 3.4, there exists a strictly order-preserving
continuous path Jω0 : [0, 1] → A(ω0) with endpoints (ω0, p(ω0)) and (ω0, q(ω0)).
By virtue of Lemma 3.5, we have
(ω0, u(ω0)) = O(ω0, u(ω0)) ∩ P−1(ω0) = Jω0(τ1)
and
(ω0, v(ω0)) = O(ω0, v(ω0)) ∩ P−1(ω0) = Jω0(τ2),
for some τ1, τ2 ∈ [0, 1]. So (ω0, u(ω0)) and (ω0, v(ω0)) are order-related, and hence
u(·) and v(·) are related w.r.t. “¿”. Thus we have proved the claim.
Based on this claim, A(ω) is totally-ordered for every ω ∈ Ω. By repeating the
arguments for proving (3.4) in Lemma 3.4, we further deduce that A(ω) is closed,
and hence, A(ω) is locally compact because Πt is fiber-compact. Thus, one can
obtain that A(ω) is connected by repeating the proof in (iii) of Lemma 3.4. It then
follows from Propositions Y1-Y2 in [36, Page 434, Appendix] that for every ω ∈ Ω,
A(ω) is either a singleton or coincides with the image of a strictly order-preserving
continuous path
(3.7) Jω : Iω → A(ω),
where the interval Iω = [0, 1], [0,+∞), (−∞, 0] or (−∞,+∞). Here Iω = [0, 1]
corresponds to the case that A(ω) is bounded; Iω = [0,+∞) (resp. (−∞, 0]) cor-
responds to the case that A(ω) is unbounded but lower-bounded (resp. upper-
bounded); while Iω = (−∞,+∞) corresponds to the case that A(ω) is totally
unbounded.
Now fix an ω0 ∈ Ω and let the interval I = Iω0 . Define the mapping
(3.8) h : Ω× I → A; (ω, α) 7→ O(Jω0(α)) ∩ P−1(ω),
where Jω0 comes from (3.7) with ω replaced by ω0. It is easy to see that h is well-
defined on Ω× I. In the following, we will show that h satisfies all the statements
in Theorem 3.1:
We first note that h is surjective from Ω× I onto A. Indeed, for any (ω, x) ∈ A,
it follows from Lemma 2.3 that there exists some b(·) ∈ C(Π) such that O(ω, x) ∩
14 FENG CAO, MATS GYLLENBERG, AND YI WANG
P−1(ω∗) = (ω∗, b(ω∗)), for all ω∗ ∈ Ω. In particular, x = b(ω). Note also
that (ω0, b(ω0)) ∈ A(ω0). Then, by (3.7), one can find an α ∈ I such that
Jω0(α) = (ω0, b(ω0)). Choose a sequence sn → +∞ such that ω0 · sn → ω.
Then Πsn(Jω0(α)) = Πsn
(ω0, b(ω0)) = (ω0 · sn, b(ω0 · sn)) → (ω, b(ω)) = (ω, x),
as n → +∞. Thus, (ω, x) ∈ O(Jω0(α))∩P−1(ω), which implies that h is surjective.
Moreover, h is also injective. In fact, let (ωi, αi) ∈ Ω×I, i = 1, 2, with h(ω1, α1) =
h(ω2, α2). By Lemma 2.3, O(Jω0(αi)) is a 1-cover of Ω, denoted by ci(·) ∈ C(Π), for
i = 1, 2. Moreover, (ω0, ci(ω0)) = Jω0(αi) since Jω0(αi) ∈ A(ω0), for i = 1, 2. Note
also that (ω1, c1(ω1)) = h(ω1, α1) = h(ω2, α2) = (ω2, c2(ω2)). Then ω1 = ω2 , ω∗
and c1(ω∗) = c2(ω∗). Since Ω is minimal, there is a sequence tn → ∞ such that
ω∗ · tn → ω0. Then Πtn(ω∗, ci(ω∗)) = (ω∗ · tn, ci(ω∗ · tn)) → (ω0, ci(ω0)) = Jω0(αi)
as n → ∞, for i = 1, 2. Recall that c1(ω∗) = c2(ω∗), then we obtain Jω0(α1) =
Jω0(α2), which implies that α1 = α2. Thus, h is also injective.
In order to prove that h is continuous, we choose any sequence (ωk, αk)∞k=1 ⊂Ω × I with (ωk, αk) → (ω∞, α∞) as k → ∞. Again by Lemma 2.3, for each
k = 1, 2, · · · ,∞, there exists an ak(·) ∈ C(Π) such that
(3.9) h(ω, αk) = O(Jω0(αk)) ∩ P−1(ω) = (ω, ak(ω)), ∀ω ∈ Ω.
In particular, (ω0, ak(ω0)) = Jω0(αk), for k = 1, 2, · · · ,∞. Recall that αk → α∞ as
k →∞, then one has ak(ω0) → a∞(ω0) as k →∞. It then follows from assumption
(H3) that, for any ε > 0, there exists a positive integer K ∈ N such that
‖(ω0 · t, ak(ω0 · t))− (ω0 · t, a∞(ω0 · t))‖ = ‖Πt(ω0, ak(ω0))−Πt(ω0, a∞(ω0))‖ < ε/3,
for all k ≥ K and t ≥ 0. This implies that, if k ≥ K then
(3.10) ‖ak(ω)− a∞(ω)‖ < ε/3,
uniformly for all ω ∈ Ω. Moreover, for such ε and K (choose K larger if necessary),
it is easy to see that
(3.11) ‖ωk − ω∞‖ < ε/3 and ‖a∞(ωk)− a∞(ω∞)‖ < ε/3,
for all k ≥ K. By virtue of (3.9), (3.10) and (3.11), we have
‖h(ωk, αk)− h(ω∞, α∞)‖ = ‖(ωk, ak(ωk))− (ω∞, a∞(ω∞))‖≤ ‖ωk − ω∞‖+ ‖ak(ωk)− a∞(ωk)‖+ ‖a∞(ωk)− a∞(ω∞)‖< ε/3 + ε/3 + ε/3 = ε,
for all k ≥ K. We have proved that h is continuous.
COMPARABLE SKEW-PRODUCT SEMIFLOWS 15
Now we will show that h satisfies (i)-(v).
Statement (i) is obvious because O(Jω0(α)) is a 1-cover of Ω for each α ∈ I.
(ii): Since h is surjective, h(ω, I) = A(ω) for each ω ∈ Ω. Let I be nontrivial
and assume α1, α2 ∈ I with α1 < α2. By the monotonicity of Jω0(·) and Πt, one
has h(ω, α1) ≤ h(ω, α2). Note also that h is injective. We obtain that h(ω, α1) <
h(ω, α2), and hence, h(ω, α1) ¿ h(ω, α2) by the eventually strong monotonicity of
Πt.
According to the statement right after (3.7), in order to prove (iii)-(v), it suffices
to prove that
(3.12) Iω ≡ I, for all ω ∈ Ω.
To end this, suppose that there is an ω∗ ∈ Ω such that Iω∗ 6= I(= Iω0). We now
assert that A(ω0) is upper-bounded (resp. lower-bounded) if and only if A(ω∗) is
upper-bounded (resp. lower-bounded). (We only prove the “upper-bounded” case.
The other case is similar). Indeed, if A(ω0) is upper-bounded, then (ω0, w) =
l.u.b.A(ω0) exists because A(ω0) is totally-ordered. Note also that A(ω0) is closed.
Then (ω0, w) ∈ A(ω0). Let z ∈ X be such that (ω∗, z) = O(ω0, w) ∩ P−1(ω∗).
Then (ω∗, z) ∈ A(ω∗). Moreover, A(ω∗) is upper-bounded by (ω∗, z) (Otherwise,
one can find (ω∗, z′) ∈ A(ω∗) such that (ω∗, z) < (ω∗, z′). By the eventually strong
monotonicity, (ω0, w) < O(ω∗, z′)∩P−1(ω0) ∈ A(ω0), contradicting to the definition
of w). Noticing that ω0 and ω∗ are symmetric, we have proved the assertion.
Based on this assertion, it is easy to see that A(ω0) is bounded (or unbounded
but lower-bounded or unbounded but upper-bounded, or totally unbounded) if and
only if A(ω∗) is of the same type. This implies that Iω0 = Iω∗ , a contradiction.
Consequently, Iω ≡ Iω0 = I for all ω ∈ Ω. Therefore, the statements in (iii)-(v)
have been obtained. In particular, by the continuity of h on Ω× I, A is compact if
I = 0 or [0, 1]. Thus, we have completed the proof. ¤
4. Abstract results for comparable systems
Definition 4.1 (Comparable skew-product semiflow). A skew-product semiflow
Γt(ω, x) = (ω · t, v(t, ω, x)) on Ω × X is called lower-comparable (resp. upper-
comparable) with respect to Πt, if Γt(ω, x) ≥ Πt(ω, y) (resp. Γt(ω, x) ≤ Πt(ω, y))
whenever (ω, x), (ω, y) ∈ Ω×X with (ω, x) ≥ (ω, y) (resp. (ω, x) ≤ (ω, y)).
In this section, we will establish the 1-cover property for the ω-limit sets of the
comparable skew-product semiflows Γt.
16 FENG CAO, MATS GYLLENBERG, AND YI WANG
Lemma 4.2. Assume that (H1)-(H3) hold. Assume also a skew-product semiflow
Γt is lower-comparable (resp. upper-comparable) with respect to Πt. Let K ⊂Ω ×X be an ω-limit set of Γt. Then there exist a 1-cover a∗(·) ∈ C(Π) such that
(ω, a∗(ω)) ≤ K ∩ P−1(ω) (resp. (ω, a∗(ω)) ≥ K ∩ P−1(ω)), for all ω ∈ Ω.
Proof. We only prove the case that Γt is lower-comparable w.r.t. Πt. The other
case is similar.
Since K is an ω-limit set of Γt, one can write K = OΓ(ω0, x0) for some (ω0, x0) ∈Ω×X. For such (ω0, x0), it follows from Lemma 2.3 that there exists some a∗(·) ∈C(Π) such that OΠ(ω0, x0)∩P−1(ω) = (ω, a∗(ω)), for all ω ∈ Ω. Then we claim that
(ω, a∗(ω)) ≤ K∩P−1(ω) for all ω ∈ Ω. As a matter of fact, for any (ω, y) ∈ K, there
exists a sequence tn → ∞ such that Γtn(ω0, x0) → (ω, y) as n →∞. By taking a
subsequence, if necessary, one may assume that Πtn(ω0, x0) → (ω, z) ∈ OΠ(ω0, x0).
It then follows from the 1-cover property of OΠ(ω0, x0) that a∗(ω) = z. Note
also that Γt is lower-comparable w.r.t. Πt. Then one has a∗(ω) = z ≤ y. Since
(ω, y) ∈ K is arbitrarily chosen, we obtain that (ω, a∗(ω)) ≤ K ∩ P−1(ω) for all
ω ∈ Ω, which completes the proof. ¤
Theorem 4.3. Assume that a skew-product semiflow Γt is lower-comparable (resp.
upper-comparable) with respect to Πt. Let K ⊂ Ω×X be the ω-limit set of a forward
orbit O+(ω0, x0) of Γt. Then we have the following:
(i) If there exists an ω∗ ∈ Ω such that K ∩ P−1(ω∗) ≤ (ω∗, b) (resp. K ∩P−1(ω∗) ≥ (ω∗, b)) for some (ω∗, b) ∈ A(ω∗), then K contains a unique minimal
set M . Moreover, M is an almost 1-cover of Ω w.r.t. Γt.
(ii) For any ω ∈ Ω, if there exists some (ω, b) ∈ A(ω) such that K ∩ P−1(ω) ≤(ω, b) (resp. K ∩ P−1(ω) ≥ (ω, b)), then K is a 1-cover of Ω w.r.t. Γt.
(iii) If I = [0,+∞) or (−∞,+∞) (resp. I = (−∞, 0] or (−∞,+∞)) in Theorem
3.1 for Πt, then K is a 1-cover of Ω w.r.t. Γt.
Proof. We only prove the case that Γt is lower-comparable w.r.t. Πt. The other
case is similar.
(i) By virtue of Lemma 4.2, there exists a 1-cover a∗(·) ∈ C(Π) such that
(ω, a∗(ω)) ≤ K∩P−1(ω) for all ω ∈ Ω. Hence, we define a nonempty set Y ⊂ C(Π),
for which
(4.1) Y = y(·) ∈ C(Π) : (ω, y(ω)) ≤ K ∩ P−1(ω), for all ω ∈ Ω.
COMPARABLE SKEW-PRODUCT SEMIFLOWS 17
Recall that Πt is monotone and A(ω) is totally ordered for all ω ∈ Ω (by Theorem
3.1). Then C(Π) is also totally-ordered with respect to ¿. Accordingly, l.u.b.Yexists. Moreover, it is also easy to check that l.u.b.Y ∈ Y.
Denote p(·) = l.u.b.Y. Now we claim that there exists an ω ∈ Ω such that K ∩P−1(ω) = (ω, p(ω)). Suppose on the contrary that, for any ω ∈ Ω, there is some
(ω, xω) ∈ K such that (ω, p(ω)) < (ω, xω). Then, by the strong monotonicity of Πt
and the comparability of Γt w.r.t. Πt, one can assume without loss of generality
that (ω, p(ω)) ¿ (ω, xω) for all ω ∈ Ω. In particular, for the ω∗ ∈ Ω given in our
assumption, one has
(4.2) (ω∗, p(ω∗)) ¿ (ω∗, xω∗) ≤ (ω∗, b(ω∗)).
For such p(·) and b(·), it follows from Lemma 3.4 that there exists a strictly order-
preserving continuous path Jω∗ : [0, 1] → A(ω∗) with endpoints Jω∗(0) = (ω∗, p(ω∗))
and Jω∗(1) = (ω∗, b(ω∗)). As a consequence, one can find some q(·) ∈ C(Π) such
that (ω∗, p(ω∗)) ¿ (ω∗, q(ω∗)) ¿ (ω∗, xω∗). Since K is the ω-limit set of (ω0, x0)
with respect to Γt, there exists some sequence tn → +∞ such that
Γtn(ω0, x0) → (ω∗, xω∗) ∈ K as n →∞.
By choosing a subsequence, if necessary, we also obtain that
Πtn(ω0, q(ω0)) → (ω∗, q(ω∗)) as n →∞.
Accordingly, there exists N À 1 such that
ΠtN(ω0, q(ω0)) ¿ ΓtN
(ω0, x0).
Again, from the eventually strong monotonicity of Πt and the comparability of Γt
w.r.t. Πt, it follows that
(4.3) Πt+tN(ω0, q(ω0)) ¿ ΠtΓtN
(ω0, x0) ≤ Γt+tN(ω0, x0), ∀t ≥ 0.
For any (ω, x) ∈ K, there exists sn → +∞ such that
Γsn(ω0, x0) → (ω, x) as n →∞.
Given such sequence sn, let t = sn − tN in (4.3) for all n sufficiently large. This
implies that
Πsn(ω0, q(ω0)) ¿ Γsn
(ω0, x0)
18 FENG CAO, MATS GYLLENBERG, AND YI WANG
for all n sufficiently large. By Letting n → ∞, it yields that (ω, q(ω)) ≤ (ω, x).
Since (ω, x) ∈ K is arbitrary, we obtain that (ω, q(ω)) ≤ K ∩P−1(ω) for all ω ∈ Ω,
contradicting the definition of p(·). Thus we have proved the claim.
Using this claim, we deduce that there is a unique minimal set M ⊂ K. Other-
wise, suppose on the contrary that K contains two distinct minimal sets M1 and M2,
then it follows from the minimality of Ω that Mi ∩P−1(ω) are nonempty, for every
ω ∈ Ω and i = 1, 2. In particular, ∅ 6= Mi ∩ P−1(ω) ⊂ K ∩ P−1(ω) = (ω, p(ω)),for i = 1, 2. Hence M1 ∩ P−1(ω) = M2 ∩ P−1(ω) = (ω, p(ω)). This implies that
M1 = M2, a contradiction. Thus, K only contains a unique minimal set M , with
M ∩ P−1(ω) = (ω, p(ω)). By [41, Definition 1.2.11 and Corollary 1.2.15], M is
an almost automorphic extension of Ω, i.e., M is an almost 1-cover of Ω. Thus we
have proved the first statement.
(ii) Assume that for any ω ∈ Ω, there exists some (ω, b) ∈ A(ω) such that
K ∩ P−1(ω) ≤ (ω, b). Then we assert that K is a 1-cover of Ω w.r.t. Γ, satisfying
K∩P−1(ω) = (ω, p(ω)) for ω ∈ Ω. Here p(·) = l.u.b.Y is defined in (4.1). Otherwise,
there exist an ω1 ∈ Ω and some (ω1, c) ∈ K ∩ P−1(ω1) such that (ω1, p(ω1)) <
(ω1, c). Let ω2 = ω1 ·t1 for some t1 > t0 > 0. By the eventually strong monotonicity
of Πt and the comparability of Γt w.r.t. Πt, one has (ω2, p(ω2)) ¿ Γt1(ω1, c) ∈ K ∩P−1(ω2). According to our assumption, for such ω2, we can find (ω2, b(ω2)) ∈ A(ω2)
such that
(ω2, p(ω2)) ¿ Γt1(ω1, c) ≤ (ω2, b(ω2)),
which is exactly (4.2), with ω∗ replaced by ω2, respectively. Hence, by repeating
the same arguments following (4.2) in the proof of (i), one obtains a contradiction
to the definition of p(·). So, we have proved the assertion, which completes our
proof of (ii).
(iii) Finally, assume that I = [0,+∞) or (−∞,+∞) in Theorem 3.1 for Πt. Then,
by Theorem 3.1(iv)-(v), A(ω) is upper-unbounded for any ω ∈ Ω. Note also that
K is bounded. Consequently, for every ω ∈ Ω, there exists some (ω, b(ω)) ∈ A(ω)
such that K ∩P−1(ω) ≤ (ω, b(ω)). Thus, (iii) is a direct corollary of statement (ii).
We have completed our proof. ¤
5. Asymptotic almost periodicity in comparable systems
It is well known that there are many differential equations generate monotone
skew-product semiflows satisfying (H1)-(H3) (cf. [23, 24, 35, 41] and references
therein). As a consequence, our results have wide applications to various types of
COMPARABLE SKEW-PRODUCT SEMIFLOWS 19
comparable nonmonotone systems of differential equations. We do not intend to
present them all, but give four typical examples to illustrate how our main theorems
are applied to study the asymptotic almost periodicity of solutions to nonmonotone
comparable almost periodic ODEs, reaction-diffusion systems, delayed differential
systems and semilinear parabolic equations.
5.1. Almost periodic comparable ODE systems. Let Rn+ = (x1, ..., xn) ∈
Rn : xi ≥ 0,∀1 ≤ i ≤ n. Consider the n-dimensional system of ordinary differential
equations
(5.1) x = f(t, x),
where f satisfies the following hypotheses:
(A1) f ∈ C1(R× Rn+,Rn) is C1-admissible and uniformly almost periodic in t;
(A2) fi(t, 0) = 0 (1 ≤ i ≤ n);
(A3) (Cooperativity and Strong Irreducibility) ∂fi
∂xj(t, x) ≥ 0 for all 1 ≤ i 6= j ≤ n.
Moreover, there is a δ > 0 such that if two nonempty subsets I, J of 1, 2, · · · , nform a partition of 1, 2, · · · , n, then for any (t, x) ∈ R × Rn
+, there exist i ∈ I,
j ∈ J such that | ∂fi
∂xj(t, x)| ≥ δ > 0;
(A4) There is a C1-function L : Rn+ → R such that gradL(x) À 0 at each x ∈ Rn
+,
and 〈gradL(x), f(t, x)〉 = 0 for all (t, x) ∈ R× Rn+.
Hypothesis (A4) says that L(x) has positive gradient at each x ∈ Rn+ and is a first
integral of system (5.1), i.e., L(x) is a constant along every solution of (5.1). There
are many models possessing such invariant L, such as gross-substitute systems in
economics (cf. [32, 40]) and master equation in Markov processes ([13, 27]), etc.
Let H(f) be the hull of f . Then it is easy to check that each g ∈ H(f) satisfies
(A1)-(A4) as well. Note that the time translation g ·t of g ∈ H(f) induces a natural
almost periodic minimal flow on H(f). It then follows that system (5.1) induces
the following (local) skew-product flow:
Πt : H(f)× Rn+ → H(f)× Rn
+; (g, x) 7→ (g · t, φ(t, x; g)), t ∈ R,
where φ(t, x; g) is the solution of
(5.1g) x = g(t, x)
with φ(0, x; g) = x. By virtue of (A3), Πt is eventually strongly monotone (see [41,
Lemma 3.4.5]). Assume also that
20 FENG CAO, MATS GYLLENBERG, AND YI WANG
(A5) Every solution of (5.1g), g ∈ H(f), is bounded.
Then (A4) implies that every forward orbit of Πt is uniformly stable (see [42, Lemma
3.1]). Thus, Hypotheses (H1)-(H3) are satisfied for Πt.
Now we consider the following nonmonotone system
(5.2) x = F (t, x),
where F satisfies (A1) and its frequency module M(F ) = M(f). Moreover, we
assume that either
(5.2-UC) Fi(t, x) ≥ 0 if xi = 0, and F (t, x) ≤ f(t, x) for all (t, x) ∈ R× Rn+;
or
(5.2-LC) F (t, x) ≥ f(t, x) for all (t, x) ∈ R× Rn+.
Then system (5.2) generates a (local) skew-product flow on H(F )× Rn+:
Γt : H(F )× Rn+ → H(F )× Rn
+; (G, x) 7→ (G · t, ψ(t, x;G)), t ∈ R,
where ψ(t, x;G) is the solution of
x = G(t, x)
with G ∈ H(F ) and ψ(0, x;G) = x.
Theorem 5.1. Let (A1)-(A5) hold. Assume also either (5.2-UC) or (5.2-LC) is
satisfied. Then every bounded solution of the comparable system (5.2) will be as-
ymptotic to an almost periodic solution.
Proof. Note that M(F ) = M(f), as we mentioned in the end of Section 2, the flow
(H(F ), ·) is isomorphic to the flow (H(f), ·), and hence one may write Ω := H(f) ∼=H(F ). By virtue of Kamke Theorem (cf. [26, 31, 23]), we further obtain that Γt is
upper-comparable with respect to Πt (when (5.2-UC) holds); or lower-comparable
with respect to Πt (when (5.2-LC) holds).
Let K be the ω-limit set of a bounded solution of the comparable system (5.2).
(i) Assume that (5.2-UC) holds, i.e., Γt is upper-comparable with respect to Πt.
Note that K ⊂ Ω×Rn+, and 0(·) ∈ C(Π) by (A2). Then K∩P−1(ω) ≥ (ω, 0) ∈ A(ω)
for any ω ∈ Ω. By Theorem 4.3(ii), we obtain that K is a 1-cover of Ω w.r.t. Γt.
(ii) Assume that (5.2-LC) holds, i.e., Γt is lower-comparable with respect to Πt.
We will utilize Theorem 4.3(iii) to deduce the 1-cover property of K. Accordingly,
it suffices to show that I = [0,+∞) in Theorem 3.1. By virtue of Theorem 3.1(iv),
COMPARABLE SKEW-PRODUCT SEMIFLOWS 21
this corresponds to that A(ω) is lower-bounded but upper-unbounded, for all ω ∈ Ω.
By (A2), it is easy to see that A(ω) is lower-bounded. So, we only need to show
that A(ω) is upper-unbounded for all ω ∈ Ω. Suppose on the contrary that A(ω0)
is upper-bounded for some ω0 ∈ Ω. Then (ω0, w) := l.u.b.A(ω0) exists because
A(ω0) is totally-ordered (see Theorem 3.1). Note also that A(ω0) is closed. Then
(ω0, w) ∈ A(ω0). Now choose z ∈ Rn+ such that w ¿ z. Then
Πt(ω0, w) ¿ Πt(ω0, z), for all t ≥ 0.
Recall that (ω0, w) ∈ A(ω0), we choose a sequence tn →∞ such that ω0 · tn → ω0
and φ(tn, w;ω0) → w as n →∞. For such tn, choose a subsequence, if necessary,
such that φ(tn, z;ω0) → z∗ as n →∞. By Lemma 2.3 and monotonicity of Πt, one
has (ω0, z∗) ∈ A(ω0) and w ≤ z∗. On the other hand, it follows from (A4) that
L(w) = L(φ(t, w;ω0)) < L(φ(t, z;ω0)) = L(z∗) for all t ≥ 0. Then w < z∗, which
contradicts to the definition of w. Thus, we have proved that I = [0,+∞) holds in
our case. By Theorem 4.3(iii), we obtain that K is a 1-cover of Ω w.r.t. Γt. ¤
5.2. Almost periodic comparable reaction-diffusion systems. Consider the
almost periodic reaction-diffusion system with Neumann boundary condition:
(5.3)
∂ui
∂t= di(t)∆ui + Fi(t, x, u1, · · · , un), x ∈ Ω, t > 0,
∂ui
∂ν(t, x) = 0, x ∈ ∂Ω, t > 0,
ui(0, x) = u0,i(x), x ∈ Ω, 1 ≤ i ≤ n,
where Ω is a bounded domain in RN with smooth boundary. Of course, ∆ is the
Laplacian operator on RN .
Let d = (d1(·), · · · , dn(·)) ∈ C(R,Rn) be an almost periodic vector-valued func-
tion bounded below by a positive real vector. The nonlinearity F = (F1, · · · , Fn) :
R × Ω × Rn → Rn is a C1-admissible (with D = Ω × Rn ⊂ RN+n) and uniformly
almost periodic in t, vector-valued function. Let u = (u1, · · · , un), X = C(Ω,Rn)
and the standard cone X+ = C(Ω,Rn+), together with the requirement that F
points into X+ along the boundary of X+:
(B1) Fi(t, x, u) ≥ 0, for any u ∈ X+ with ui = 0, and x ∈ Ω, t ∈ R+.
Here we do not assume F has any monotonicity properties.
Let Y = H(d, F ) be the hull of the function (d, F ). By the standard theory
of reaction-diffusion systems (see, e.g., [18], Chapter 6), it follows that for every
22 FENG CAO, MATS GYLLENBERG, AND YI WANG
u0 ∈ X+ and y = (µ,G) ∈ Y , the system
(5.4)
∂ui
∂t= µi(t)∆ui + Gi(t, x, u), x ∈ Ω, t > 0,
∂ui
∂ν(t, x) = 0, x ∈ ∂Ω, t > 0,
u(0, x) = u0(x), x ∈ Ω, 1 ≤ i ≤ n
admits a (locally) unique regular solution u(t, x, u0, y) in X+. This solution also
continuously depends on y ∈ Y and u0 ∈ X+ (see, e.g., [16, Sec.3.4]). Therefore,
(5.4) defines a (local) skew-product semiflow Γ on X+ × Y with
Γt(u0, y) = (u(t, ·, u0, y), y · t), ∀ (u0, y) ∈ X+ × Y, t ≥ 0.
Now we assume that there exists a vector-valued function f : R × Rn+ → Rn
satisfying (A1)-(A5) in Section 5.1, with its frequency module M(f) = M(F ),
such that either
(B2+) F (t, x, u) ≤ f(t, u) for all (t, x, u) ∈ R× Ω× Rn+;
or
(B2−) F (t, x, u) ≥ f(t, u) for all (t, x, u) ∈ R× Ω× Rn+.
We consider the hull H(d, f). Since M(f) = M(F ), one has Y = H(d, F ) ∼=H(d, f). Then it is easy to see that, for any (µ,G) ∈ H(d, F ), there exists a
(µ, g) ∈ H(d, f) such that either
(i) G(t, x, u) ≤ g(t, u) for all (t, x, u) ∈ R× Ω× Rn+ (when (B2+) holds); or
(ii) G(t, x, u) ≥ g(t, u) for all (t, x, u) ∈ R× Ω× Rn+ (when (B2−) holds).
For such (µ, g) ∈ H(d, f), we introduce the following new reaction-diffusion
system:
(5.5)
∂vi
∂t= µi(t)∆vi + gi(t, u), x ∈ Ω, t > 0,
∂vi
∂ν(t, x) = 0, x ∈ ∂Ω, t > 0,
v(0, x) = v0(x) ∈ X+, x ∈ Ω, 1 ≤ i ≤ n.
Then system (5.5) induces the following global skew-product semiflow:
Πt : X+×H(d, f) → X+×H(d, f); (v0, (µ, g)) 7→ (v(t, ·, v0;µ, g), (µ, g)·t), t ∈ R+,
where v(t, ·, v0;µ, g) is the unique regular global solution of (5.5) in X+. According
to assumption (A3) and [24, Sec.6.2], one can obtain that Πt is fibre-compact and
satisfies standard hypotheses (H1)-(H3).
COMPARABLE SKEW-PRODUCT SEMIFLOWS 23
Lemma 5.2. (i) If (B2+) holds, then Γt is upper-comparable with respect to Πt;
(ii) If (B2−) holds, then Γt is lower-comparable with respect to Πt.
Proof. We only prove (i). The proof of (ii) is similar. To end this, we follow the
comparison arguments in [9]. Let u0, v0 ∈ X+ with u0 ≤ v0, and define the function
w by
w(t, x) = v(t, x, v0;µ, g)− u(t, x, u0;µ,G).
Then w satisfies
(5.6)
∂wi
∂t= µi(t)∆wi + Qi(t, x, w), x ∈ Ω, t > 0,
∂wi
∂ν(t, x) = 0, x ∈ ∂Ω, t > 0,
w(0, x) = w0(x) ≥ 0, x ∈ Ω, 1 ≤ i ≤ n,
where
Qi(t, x, w) = gi(t, w + u(t, x, u0;µ,G))−Gi(t, x, u(t, x, u0;µ,G)),
for each 1 ≤ i ≤ n. We claim that the rectangle Rn+ is invariant for Q = (Q1, · · · , Qn).
From [18, Proposition 6.2], it follows that we only need to show that for any i,
Qi(t, x, w) ≥ 0 if w ≥ 0 with wi = 0. Indeed, recall that g is cooperative. Then
gi(t, w + u) = gi(t, w1 + u1, · · · , ui, · · · , wn + un)
≥ gi(t, u1, · · · , ui, · · · , un) = gi(t, u) ≥ Gi(t, x, u),
for any (t, x, u) ∈ R+ × Ω × Rn+. As a consequence, it follows that Qi(t, x, w) ≥ 0
if w ≥ 0 with wi = 0. Thus we complete the proof. ¤
In the following, we write (B2±) as either (B2+) or (B2−) holds.
Theorem 5.3. Let (B1) and (B2±) hold for system (5.3). Then every L∞-bounded
solution of (5.3) is asymptotic to an almost periodic and spatially homogeneous
solution.
Proof. Let u(t, ·, u0, d, F ) be a L∞-bounded solution of (5.3) in X+. Then, following
from the work in [16] and the standard a priori estimates for parabolic equations,
we know that the solution u becomes a globally defined classical solution on X+;
moreover, one obtains that u(t, ·, u0, d, F ) : t ≥ τ is precompact in X+. Thus the
ω-limit set of (u0, (d, F )) = (u0, y) ∈ X+ × Y , with respect to Γt, is a nonempty
and compact set in X+. We denote such ω-limit set by K.
24 FENG CAO, MATS GYLLENBERG, AND YI WANG
(i) If (B2+) holds, then it follows from Lemma 5.2 that Γt is upper-comparable
with respect to Πt. Note that K ⊂ X+ × Y , and 0(·) ∈ C(Π) by (A2). Then
K ∩ P−1(y) ≥ (0, y) ∈ A(y) for any y ∈ Y . By Theorem 4.3(ii), we obtain that K
is a 1-cover of Ω w.r.t. Γt.
(ii) if (B2−) holds, then Γt is lower-comparable with respect to Πt. Again, we
will utilize Theorem 4.3(iii) to deduce the 1-cover property of K. Accordingly, it
suffices to show that I = [0,+∞) in Theorem 3.1. By virtue of Theorem 3.1(iv), this
corresponds to that A(y) is lower-bounded but upper-unbounded, for all y ∈ Y . By
[24, Theorem 6.3], we note that A ⊂ Rn+ × Y . Then one can repeat the arguments
in Case (5.2-LC) in Section 5.1 to obtain that I = [0,+∞) holds in our case. It
then follows from Theorem 4.3(iii) that K is a 1-cover of Ω w.r.t. Γt. ¤
5.3. Almost periodic comparable delayed differential systems. Consider
an almost periodic n-compartmental system with pipes describing by the following
differential systems with time delays (see e.g., [6, 24, 51]):
(5.7)
dui(t)dt
=n∑
j=1
fij(t− τij , uj(t− τij))−n∑
j=1
fji(t, ui(t)), t > 0, 1 ≤ i ≤ n,
u(s) = ϕ(s), s ∈ [−τ, 0],
where τij ≥ 0, τ = max1≤i,j≤nτij and ϕ ∈ X , C([−τ, 0],Rn). Obviously, X is
a strongly ordered Banach space with solid cone X+ = C([−τ, 0],Rn+). We assume
that
(C1) Each fij ∈ C(R2,R) is C1-admissible and uniformly almost periodic in t;
(C2) fij(t, 0) ≡ 0, for all t ∈ R and 1 ≤ i, j ≤ n;
(C3) There exists a δ > 0 such that ∂fij
∂u (t, u) ≥ δ > 0, ∀(t, u) ∈ R2, 1 ≤ i, j ≤ n.
Let f = (fij)1≤i,j≤n and let H(f) be the hull of f . For each (g, ϕ) ∈ H(f)×X,
let u(t, g, ϕ) be the unique solution of (5.7g) (i.e., (5.7) with f replaced by g), with
the initial function ϕ ∈ X. Then system (5.7) generates a skew-product semiflow:
Πt : H(f)×X → H(f)×X; (g, ϕ) 7→ (g · t, ut(g, ϕ)), t ≥ 0,
where
[ut(g, ϕ)](s) = u(t + s, g, ϕ), ∀s ∈ [−τ, 0].
COMPARABLE SKEW-PRODUCT SEMIFLOWS 25
Now we consider the nonmonotone system:
(5.8)
dui(t)dt
=n∑
j=1
Fij(t− τij , uj(t− τij))−n∑
j=1
fji(t, ui(t)), t > 0, 1 ≤ i ≤ n,
u(s) = ϕ(s), s ∈ [−τ, 0],
for which F = (Fij)1≤i,j≤n satisfies (C1) and its frequency M(F ) = M(f) (hence
(H(F, f), ·) is flow isomorphic to the flow (H(f), ·)). We also assume that either
(5.8-UC) Fij(t, u) ≤ fij(t, u) for all (t, u) ∈ R2 and 1 ≤ i, j ≤ n;
or
(5.8-LC) Fij(t, u) ≥ fij(t, u) for all (t, u) ∈ R2 and 1 ≤ i, j ≤ n.
Denote Ω := H(F, f) ∼= H(f). For each (ψ, ω) ∈ X × Ω, we write v(t, ψ, ω) the
unique solution of (5.8ω) (i.e., (5.8) with (F, f) replaced by ω), with the initial
function ψ ∈ X. System (5.8) generates a (local) skew-product semiflow on X ×Ω:
Γt : Ω×X → Ω×X; (ω, ψ) 7→ (ω · t, vt(ψ, ω)), t ≥ 0,
where [vt(ω, ψ)](s) = v(t + s, ω, ψ),∀s ∈ [−τ, 0].
Theorem 5.4. Let (C1)-(C3) hold. Assume also that (5.8-LC) or (5.8-UC) is satis-
fied. Then every bounded solution of the comparable system (5.8) will be asymptotic
to an almost periodic solution.
Proof. It follows from (C3) that Πt is eventually strongly monotone (see [6, 51]).
Recall that system (5.7) possesses a first integral J : H(f)×X → R defined by (see
[24, Lemma 6.2]):
J(g, ϕ) :=n∑
i=1
ϕi(0) +n∑
i,j=1
∫ 0
−τij
gij(s, ϕj(s))ds.
J is also strictly order-preserving in the sense that
(5.9) J(g, ϕ) < J(g, ψ), for all ϕ < ψ in X and g ∈ H(f).
Then by [24, Lemmas 6.3 and 6.4], (H2) holds and Πt is uniformly stable. Thus
Πt satisfies hypotheses (H1)-(H3). Furthermore, it follows from [15, Theorem 4.1]
that Πt is fiber-compact.
For such Πt, we will show that I = (−∞ + ∞) in Theorem 3.1. By virtue of
Theorem 3.1(v), this corresponds to that A(g) is both lower-unbounded and upper-
unbounded, for all g ∈ H(f). Otherwise, suppose without loss of generality that
26 FENG CAO, MATS GYLLENBERG, AND YI WANG
A(g0) is lower-bounded for some g0 ∈ H(f). Then (g0, ϕ) := g.l.b.A(g0) ∈ A(g0),
because A(g0) is totally-ordered and closed (see Theorem 3.1). Now choose ψ ∈ X
such that ψ ¿ ϕ. Then
Πt(g0, ψ) ¿ Πt(g0, ϕ), for all t > 0 sufficiently large.
Choose a sequence tn → ∞ such that g0 · tn → g0 and utn(g0, ϕ) → ϕ as n →∞. One may also assume that utn(g0, ψ) → ψ∗ as n → ∞. By Lemma 2.3 and
monotonicity of Πt, one has (g0, ψ∗) ∈ A(g0) and ψ∗ ≤ ϕ. On the other hand, it
follows from (5.9) that J(g0, ϕ) = J(Πt(g0, ϕ)) > J(Πt(g0, ψ)) = J(g0, ψ∗) for all
t ≥ 0. Then ψ∗ < ϕ, which contradicts to the definition of ϕ. Thus, we have proved
that I = (−∞,+∞) holds in our case.
By virtue of the comparison Theorem (see [43, Corollary 5.3.4] or [23]), Γt is
upper-comparable with respect to Πt (when (5.8-UC) holds); or lower-comparable
with respect to Πt (when (5.8-LC) holds).
Recall that the interval I = (−∞,+∞). It then follows from Theorem 4.3(iii)
that every bounded solution of the comparable system (5.8) will be asymptotic to
an almost periodic solution. This completes our proof. ¤
5.4. Almost periodic comparable semilinear parabolic equations. Consider
the following initial-boundary value problem for semilinear parabolic equations
(5.10)
∂u
∂t=
N∑
i,j=1
aij(x)uxixj + f(t, x, u), x ∈ Ω, t > 0,
Bu =0, x ∈ ∂Ω, t > 0,
u(0, x) = u0(x), x ∈ Ω.
Here Ω is a bounded domain in RN (N ≥ 1) with the boundary ∂Ω of class C2+θ
for some θ ∈ (0, 1). The operator Lu :=∑
aijuxixjsatisfies uniform ellipticity
condition
(D1)∑
aij(x)ξiξj ≥ c1|ξ|2 (x ∈ Ω, ξ = (ξ1, · · · , ξN ) ∈ RN ) for some positive
constant c1 > 0.
The nonlinearity f : R × Ω × R → R is assumed to be a admissible (with
D = Ω×R ⊂ RN+1) and uniformly almost periodic in t, real-valued function with
certain regularity condition
(D2) aij ∈ Cθ(Ω), f is continuous on R × Ω × R together with its derivatives
with respect to u. Moreover, f is locally Holder continuous in (t, x).
COMPARABLE SKEW-PRODUCT SEMIFLOWS 27
We consider a time-independent regular linear boundary operator B on ∂Ω of Neu-
mann (Bu = ∂u∂ν ) or Robin (Bu = ∂u
∂ν +αu) type. Here ν is the unit outward normal
vector field on ∂Ω. We also assume that
(D3) f(t, x, 0) = 0 and there is an M > 0 such that f(t, x, u) ≤ −δ0, for any
(t, x) ∈ R× Ω and u ≥ M .
(D4) f(t, x, su) ≥ sf(t, x, u) for all s ∈ [0, 1] and (t, x, u) ∈ R× Ω× R+.
Let X = C(Ω,R). Then X is a strongly ordered Banach space with solid cone
X+ = C(Ω,R+). Let also Y = H(f) be the hull of the nonlinearity f . For any
g ∈ Y , the function g is uniformly almost periodic in t and satisfies all the above
assumptions (D1)-(D4) with the same M . As a consequence, (5.10) gives rise to a
family of equations associated to each g ∈ Y :
(5.10g)
∂u
∂t=
N∑
i,j=1
aij(x)uxixj+ g(t, x, u), x ∈ Ω, t > 0,
Bu =0, x ∈ ∂Ω, t > 0,
u(0, x) = u0(x), x ∈ Ω.
For every u0 ∈ X+, equation (5.10g) admits a (locally) unique regular solution
u(t, x, u0, g) in X+. This solution also continuously depends on g ∈ Y and u0 ∈ X+
(see e.g. [16, Sec.3.4]). Therefore, (5.10g) defines a (local) skew-product semiflow
Π on X+ × Y with
Πt(u0, g) = (u(t, ·, u0, g), g · t), ∀ (u0, g) ∈ X+ × Y, t ≥ 0.
It follows from the strong parabolic maximum principle that Πt is eventually
strongly monotone (see, e.g. [18, Theorem 6.15]). Moreover, Πt is fiber-compact
(see [18, Proposition 6.13]). (D3) implies that there is an L∞-bound on u(t, ·, u0, g) :
t ≥ 0 uniformly for u0 ∈ X+ and g ∈ Y . Once an L∞-bound is established, one
can obtain a C1-bound on u(t, ·, u0, g) : t ≥ τ for any τ > 0 (see [3, Theorem 2.4]).
Therefore, following from the work in [16] and the standard a priori estimates for
parabolic equations, we know that the solution u becomes a globally defined clas-
sical solution on X+; moreover, one gets that u(t, ·, u0, g) : t ≥ τ is precompact
for any u0 ∈ X+ and g ∈ Y .
Meanwhile, one can also deduce from (D3) that there exist minimal sets K1 =
(0, g) : g ∈ Y and K2 ⊂ [0,M ]X × Y such that
(i) K1 ≤ K2, which means that, (0, g) ≤ K2 ∩ P−1(g) for any g ∈ Y .
28 FENG CAO, MATS GYLLENBERG, AND YI WANG
(ii) For each (u0, g) ∈ X+ × Y with (u0, g) ≥ K2 ∩ P−1(g),
limt→∞
d(Πt(u0, g),K2) = 0.
For the following linearized almost periodic parabolic equation
(5.11)
∂v
∂t=
N∑
i,j=1
aij(x)vxixj+ f(t, x, 0)v, x ∈ Ω, t > 0,
Bv =0, x ∈ ∂Ω, t > 0.
According to [30, 42], there exists a unique principal spectrum point λ(f(·, ·, 0))
associated with (5.11). By [41, Proposition 2.4.1], one has λK1 = λ(f(·, ·, 0)). Here
λKi is the upper Lyapunov exponent on Ki, i = 1, 2 (see [41, Definition 2.4.3]).
Hereafter, we assume that
(D5) λK1 = λ(f(·, ·, 0)) ≤ 0.
Now we consider the following IBVP for comparable parabolic equations of the
following form
(5.12)
∂u
∂t=
N∑
i,j=1
aij(x)uxixj+ F (t, x, u), x ∈ Ω, t > 0,
Bu =0, x ∈ ∂Ω, t > 0,
u(0, x) = u0(x), x ∈ Ω,
for which (D1)-(D2) hold for aij ,F with the frequency M(F ) = M(f) (hence
(H(F ), ·) is flow isomorphic to the flow (H(f), ·)). Furthermore, we assume that
(5.12-UC) 0 ≤ F (t, x, u) ≤ f(t, x, u), ∀(t, x, u) ∈ R× Ω× R+.
Denote Y := H(F ) ∼= H(f). For each (v0, y) ∈ X+ × Y , we write v(t, v0, y)
the unique classical solution of (5.12y) (i.e., (5.12) with F replaced by y), with the
initial function v0 ∈ X+. System (5.12) generates a (local) skew-product semiflow
on X+ × Y :
Γt : X+ × Y → X+ × Y ; (v0, y) 7→ (v(t, v0, y), y · t), t ≥ 0.
Theorem 5.5. Let (D1)-(D5) and (5.12-UC) hold. Then every solution of the
comparable system (5.12) will be asymptotic to an almost periodic solution.
Proof. We first show that (D4)-(D5) implies that Πt is uniformly stable. As a
matter of fact, by virtue of (D4), Πt is subhomogeneous (or called concave) in the
COMPARABLE SKEW-PRODUCT SEMIFLOWS 29
sense that
u(t, ·, su0, g) ≥ su(t, ·, u0, g), ∀s ∈ [0, 1] and (u0, g) ∈ X+ × Y.
As a consequence, when λK1 = λ(f(·, ·, 0)) < 0, it follows from [24, Theorem 6.1(1)]
that K1 = K2 and limt→∞ d(Πt(u0, g),K1) = 0, for all (u0, g) ∈ X+×Y . So, in this
case, Πt is globally uniformly asymptotically stable. When λK1 = λ(f(·, ·, 0)) = 0,
it then follows from [33, Theorems 6.7 and 7.2] that λK2 = 0, K2 is a 1-cover of Y
and the order-interval [K1,K2] is foliated with 1-covers of Y , i.e., µK1 + (1−µ)K2
is a 1-cover, for all µ ∈ [0, 1]. Thus, Πt is uniformly stable on X+. Thus we have
shown that Πt is uniformly stable.
It follows from the Maximum Principle that Γt is upper-comparable with respect
to Πt. Let K be the ω-limit set of any solution of the comparable system (5.10).
Then, by (5.12-UC) and Lemma 4.2, one has K1 = (0, y) : y ∈ Y ≤ K ≤ K2. It
then follows from Theorem 4.3(ii) that K is a 1-cover of Y . In other words, we have
proved that every solution of the comparable system (5.10), satisfying (5.12-UC),
will be asymptotic to an almost periodic solution. ¤
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Department of Mathematics, Nanjing University of Aeronautics and Astronautics,
Nanjing, Jiangsu 210016, P. R. China
E-mail address: [email protected]
Department of Mathematics and Statistics, University of Helsinki, FIN-00014, Helsinki,
Finland
E-mail address: [email protected]
aDepartment of Mathematics and Statistics, University of Helsinki, P.O. Box 68,
FIN-00014, Finland, bDepartment of Mathematics, University of Science and Technol-
ogy of China, Hefei, Anhui 230026, P. R. China
E-mail address: [email protected], [email protected]
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