Emmy Noether, Symmetry, and Women in Mathematics
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Transcript of Emmy Noether, Symmetry, and Women in Mathematics
Emmy Noether,
Symmetry, and
Women in
Mathematics
Centre for the
Mathematics of
Symmetry and
Computation
LMS-IMA Joint
Meeting, London,
September 2018
Noether
CelebrationCheryl E
Praeger
Nathan Jacobson [edited collected papers of Emmy Noether]
Emmy Noetherโs amazing
contributions to algebra
Development of modern abstract algebra is largely due to Emmy Noether
โ โOne of the most distinctive innovations of 20th century mathematicsโ
โ Through published papers, lectures, personal influence
โ Contributions so thoroughly absorbed into math culture
โ That they are rarely specifically attributed to her
Idealtheorie in Ringbereichen Math. Annalen 83 (1921), 24-66
Ideal Theory in Rings
โข Of fundamental importance in the development of modern algebra
โข New concepts โ new framework for studying โringsโ
โข Decomposition/factorisation theorems โ general (appropriate) context
โข So fundamental, some of her ideas rediscovered by others
โข Name given to various algebraic structures: notably
โข Noetherian rings
Examples: Integers Z [positive and negative]
As well as Z: many rings had been studied
โข Ring of Gaussian integers a+bi with a, b in Z
โข Polynomial rings R[x] with coefficients in a ring R
Rings
Characteristics (axioms) of a ring R
โข Addition and multiplication possible
โข R contains zero 0 and each element a has a negative โa so a + (-a) = 0
โข Parentheses often not needed:
โข (a + b) + c = a + (b + c) and (a.b) . c = a . (b.c)
โข Distributive rule: (a + b) . c = a.c + b.c
โข Emmy Noetherโs rings were commutative: a + b = b + a and a . b = b . a
Not every ring contains a โ1โ
e.g. 2๐ = { 2 ๐ฅ | ๐ฅ โ ๐}
To convert known โdecomposition theoremsโ โ for Z or algebraic integers like Z[i], or polynomial rings Z[x]
into theorems for โidealsโ in arbitrary โintegral domainsโ (or
rings in general)
โข Analogue of prime-power decomposition of integers
โ ๐ = ๐1๐1๐2
๐2 โฆ๐๐ ๐๐ = ๐1๐2โฆ๐๐
โ Think of ๐๐ as โcomponentsโ of the decomposition
โ We know: for integers decomposition โessentially uniqueโ โ number ๐ , the
primes ๐๐, the exponents ๐๐โ Only issue is โ-1โ โ discuss later
โ Similar unique decomposition results for some rings of algebraic integers and
polynomial rings.
Emmy Noether: introduced general properties โ which could be interpreted in
all these contexts and others - to get a general decomposition theorem
whenever these properties held
Emmy Noetherโs aim in her 1921 paper
Example: 60 = 22 31 51 = 4 โ 3 โ 5
Capture the idea of prime-power decomposition for integers
Some โcharacteristic propertiesโ
In a decomposition ๐ = ๐1๐2โฆ๐๐ โข Components ๐๐ should be pairwise coprime and ๐๐ not decomposable as
product of pairwise coprime numbers (irreducible)
โข Components ๐๐ should be primary
โข If ๐๐ divides ๐๐ but not ๐ then ๐๐ divides some power of ๐
โข And two more properties โฆ
โข Properties formulated to apply to any ring:
โข โ๐ divides ๐โ means ๐ = ๐๐ for some ๐
E.g. 60 = 6 x 10
Or 60 = 2 x 2 x 3 x 5
Not allowed
E.g. 60 = 4 x 3 x 5
Component 4 is primary
Since if 4 divides ๐. ๐ but not ๐,
Then ๐ even, so 4 divides ๐2
Ideal N in a ring R
Ideals: to deal with the โuncertaintyโ
caused by โ-1โ
Definition: N subset of R such that
โข If N contains ๐ then N also contains ๐. ๐ for arbitrary ๐ in N
โข If N contains ๐ and ๐ then N also contains ๐ โ ๐
Transfers language
โข If N contains ๐ then say ๐ divisible by N
โข If M is an ideal contained in N, then say M divisible by N
Definition: N is finitely generated by subset ๐ต = {๐1, โฆ , ๐๐ }
โข if each ๐ in N can be written as ๐ = ๐1๐1 +โฏ + ๐๐ ๐๐ + ๐1๐1 +โฏ+ ๐๐ ๐๐
โข [this is modern language: Emmy Noether calls B an โideal basisโ for N]
E.g. N = 60๐ = { 60 ๐ฅ | ๐ฅ โ ๐}Is an ideal of Z
E.g. 60Z is contained in 12Z
So 60Z is divisible by 12Z
Ideal N in a ring R
Finite Chain Theorem: If every ideal is finitely generated, then
each infinite ascending chain of ideals terminates after a finite number
of steps.
Ideals: to deal with the โuncertaintyโ
caused by โ-1โ
Definition: N subset of R such that
โข If N contains ๐ then N also contains ๐. ๐ for arbitrary ๐ in N
โข If N contains ๐ and ๐ then N also contains ๐ โ ๐
Transfers language
โข If N contains ๐ then say ๐ divisible by N
โข If M is an ideal contained in N, then say M divisible by N
Definition: N is finitely generated by subset ๐ต = {๐1, โฆ , ๐๐ }
โข if each ๐ in N can be written as ๐ = ๐1๐1 +โฏ + ๐๐ ๐๐ + ๐1๐1 +โฏ+ ๐๐ ๐๐
E.g. N = 60๐ = { 60 ๐ฅ | ๐ฅ โ ๐}Is an ideal of Z
E.g. 60Z is contained in 12Z
So 60Z is โdivisible byโ 12Z
Finite Chain Theorem: If every ideal is finitely generated, then
for a countably infinite sequence of ideals such that
๐1 โ ๐2 โ โฏ โ ๐๐ โ โฏ
there exists an integer ๐ such that ๐๐ = ๐๐+1 = โฏ .
Ideals: to deal with the โuncertaintyโ
caused by โ-1โ
Finite Chain Theorem: If every ideal is finitely generated, then
for a countably infinite sequence of ideals such that
๐1 โ ๐2 โ โฏ โ ๐๐ โ โฏ
there exists an integer ๐ such that ๐๐ = ๐๐+1 = โฏ .
Ideals: to deal with the โuncertaintyโ
caused by โ-1โ
Why does it apply to the integers Z ?
โข Each ideal M is equal to M = ๐๐ = { ๐ ๐ฅ | ๐ฅ โ ๐} for some ๐
โข Each ideal N containing M is N = ๐๐ = { ๐ ๐ฅ | ๐ฅ โ ๐} for some ๐ dividing ๐
โข So M contained in only finitely many larger idealsE.g. 60๐ โ 12๐ โ 4๐ โ 2๐
Finite Chain Theorem: If every ideal is finitely generated, then
for a countably infinite sequence of ideals such that
๐1 โ ๐2 โ โฏ โ ๐๐ โ โฏ
there exists an integer ๐ such that ๐๐ = ๐๐+1 = โฏ .
Ideals: to deal with the โuncertaintyโ
caused by โ-1โ
Why does it apply to the integers Z ?
โข Each ideal M is equal to M = ๐๐ = { ๐ ๐ฅ | ๐ฅ โ ๐} for some ๐
โข Each ideal N containing M is N = ๐๐ = { ๐ ๐ฅ | ๐ฅ โ ๐} for some ๐ dividing ๐
โข So M contained in only finitely many larger ideals
Theorem was known previously for several special cases:
โข Context of modules โ Dedekind, [Supplement XI to โZahlentheorieโ]
โข Polynomial rings โ Lasker, 1905
Emmy Noether gives much wider-ranging applications of this theorem than
previously for the special cases
E.g. 60๐ โ 12๐ โ 4๐ โ 2๐
Finite Chain Theorem: If every ideal is finitely generated, then
for a countably infinite sequence of ideals such that
๐1 โ ๐2 โ โฏ โ ๐๐ โ โฏ
there exists an integer ๐ such that ๐๐ = ๐๐+1 = โฏ .
Noetherian rings
Ascending chain condition (ACC)
โข Each countably infinite ascending chain of ideals terminates after a finite
number of steps.
ACC defined also on chains of left ideals and chains of right ideals in non-
commutative rings
Definition: Ring is Noetherian if it has the ACC on left and right ideals
Next Noetherโs applications of Finite Chain Theorem: depend on Axim of Choice
Least common multiple โ. ๐.๐. {๐1, ๐2, โฆ , ๐๐} of ideals is the
intersection ๐1 โฉ ๐2 โฉโฏโฉ ๐๐
Decompositions of ideals I
Example 60๐ = 12๐ โฉ 10๐ the ideals 12๐, 10๐ the components of decomposition
Least common multiple โ. ๐.๐. {๐1, ๐2, โฆ , ๐๐} of ideals is the
intersection ๐1 โฉ ๐2 โฉโฏโฉ ๐๐
Decompositions of ideals I
Example 60๐ = 12๐ โฉ 10๐ the ideals 12๐, 10๐ the components of decomposition
Irreducible ideal: if N not the intersection (l.c.m.) of two larger ideals
So 4๐ is irreducible (only larger ideal is 2๐)
But none of 60๐ = 12๐ โฉ 10๐, 12๐ = 4๐ โฉ 3๐, or 10๐ = 2๐ โฉ 5๐ irreducible
Least common multiple โ. ๐.๐. {๐1, ๐2, โฆ , ๐๐} of ideals is the
intersection ๐1 โฉ ๐2 โฉโฏโฉ ๐๐
What about uniqueness?
Decompositions of ideals I
Example 60๐ = 12๐ โฉ 10๐ the ideals 12๐, 10๐ the components of decomposition
Irreducible ideal: if N not the intersection (l.c.m.) of two larger ideals
So 4๐ is irreducible (only larger ideal is 2๐)
But none of 60๐ = 12๐ โฉ 10๐, 12๐ = 4๐ โฉ 3๐, or 10๐ = 2๐ โฉ 5๐ irreducible
Emmyโs next theorem: [Theorem II]
Every ideal is an intersection of finitely many irreducible ideals
Always assuming:
all ideals finitely generated
Analogue of: every integer is a product of
finitely many prime-powers
Suppose ideal N = ๐1 โฉ ๐2 โฉโฏโฉ ๐๐ with the ๐๐ irreducible
Decompositions of ideals II
Shortest decomposition: cannot omit any component, that is,
No ๐๐ contains the intersection of the remaining ideals
Example: 60๐ = 4๐ โฉ 3๐ โฉ 2๐ โฉ 5๐ not shortest since 60๐ = 4๐ โฉ 3๐ โฉ 5๐ โ 2๐
NICE decomposition: N = ๐1 โฉ ๐2 โฉโฏโฉ ๐๐ if โshortestโ and all ๐๐ irreducible
Emmyโs next theorem: [Theorem IV]
Any two NICE decompositions of an ideal have same number of components
Primary and prime ideals: two definitions (products of elements
and products of ideals) โ shows equivalent
โข Some information โexponentsโ but not uniqueness [Theorem VII]
โข 11 pages [and five Theorems] refining the structure
Decompositions of ideals III
โข Primary ideals contained in unique prime ideal [Theorem V]
โข Irreducible ideals are primary [Theorem VI]
โข Hence in NICE decomposition N = ๐1 โฉ ๐2 โฉโฏโฉ ๐๐ each ๐๐ contained in
unique prime ideal ๐๐
Emmyโs next theorem: [Theorem VII]
Any two NICE decompositions involve exactly the same set of prime ideals
Section 9: Precursor of Emmy Noetherโs work on Group Rings
Study of modules
Involves
โข Extension of theory to non-commutative rings
โข Introduces concept of sub-module
โข Assuming that each submodule is finitely generated, obtains structural results
which โlay the foundations for uniqueness theoremsโ
Later work
โข 1927 Noether (her letter to Brauer)โ โI am very glad that you have now realized the connection between representation theory and
the theory of noncommutative rings โฆโ
โข 1929 Noether [Math.Z.] โ Roquet: โHer great paper on representation theory and group ringsโ
โข 1932 [J Reine und Angew. Math.] Brauer--Hasse--Noether Theorem on
division algebras over number fieldsโ Weyl: โa high watermark in the history of algebraโ
Modules are like vector spaces,
but the scalars come from a ring
(not necessarily a field)
Final applications
Section 10: Connection of her theorems with known work on polynomial rings
Section 11: Examples from Number Theory
Section 12: Example from โelementary divisor theoryโ โ matrix algebras
It is an extraordinary paper
Her influence is not just in her papers
โshe had great stimulating power and many of
her suggestions took shape only in the works
of her pupils and co-workers.โ
Hermann Weyl
Hermann Weyl:
Obituary: describes Emmy Noether as:
โIn the realm of algebra, โฆshe discovered
methods which have proved of enormous
importance in the development of the present-
day younger generation of mathematiciansโ
Albert Einstein May 1, 1935 - New York Times
โthe most significant creative mathematical genius thus far produced since the
higher education of women began.โ
For Women in Mathematics.
โข 1897 First ICM - Zurich
โ 4 women out of 208 participants
โ No women speakers
โข 1932 ICM โ again in Zurich
โ Emmy Noether first woman to give a plenary lecture at an ICM
โข 1990 ICM in Kyoto
โ Karen Uhlenbeck second woman ICM plenary speaker
โ 58 years later!
Emmy Noetherโs legacy
Emmy was first in many things.
International Congresses of Mathematicians (ICM)
Before the 1990 ICM
Women in Mathematics Associations
Associations for women in mathematics formed:
1971 Association for Women in Mathematics AWM
โ (in the US) โ with men and women as members
1980 AWM initiated: Emmy Noether Lecture
โ โto honour women who have made fundamental and sustained
contributions to the mathematical sciencesโ
โ Annually at Joint Mathematics meeting in January
โ 2013: AWM-AMS Noether lectures [co-sponsored by American Math Soc]
Additional AWM Emmy Noether lectures
โข 1994 Organised by AWM โ presented at ICM [each four years]
โข 2002 International Mathematical Union: General Assembly
โ agreed to formalize the ICM Emmy Noether Lecture
โ for following two ICMs
โข 2010 International Mathematical Union: General Assembly
โ established ICM Emmy Noether Lecture on-going
ICM Emmy Noether Lecture
1951 IMU established
โข (now) 11 persons โ President, 2 VPs, 6 Members-at-Large and immediate
past President โ 4 year terms
โข 2003-2006 First time a woman on the IMU EC:
โข Ragni Piene (Norway)
โข Ragni chaired Emmy Noether Committee for 2006
โข Members at large
โข May serve maximum of two terms
โข Terms are four years
Women and the International
Mathematical Union (IMU)
IMU Executive Committee
2007-2010 First time two women on IMU EC
Women and the International
Mathematical Union (IMU)
โ Ragni and Cheryl Praeger
โ Cheryl chaired second ICM Emmy Noether Committee - for 2010 ICM
โ ICM Emmy Noether Lecture established by IMU GA 2010
20011-2014 First woman President of IMU
Women and the International
Mathematical Union (IMU)
โ Ingrid Daubechies
2014 Established IMU Committee
โ Committee for Women in Mathematics CWM
And exceptionally โฆ..
2014 International Congress of Mathematicians in Seoul
โข Awarded by the IMU at the ICM
โข Up to four โ every four years โ for outstanding achievements by
mathematicians under 40
โข First FM awarded in 1936
โข Thereโs a long way to go
Women and the International
Mathematical Union (IMU)
Only Woman to receive a Fields Medal - in more than 80 years!