A Mathematician's Knowledge

[mathwonk]

happy birthday Gib Z! I first read Lincoln Barnett's "The universe and Doctor Einstein" at about 15, but knew nothing at all of calculus, so you are way ahead of me.

 

[Gib Z]

Well then its nice to know I have a small chance of one day becoming as knowledgeable as yourself :D

Ahh feels a little lonely to be on a physics forum on the morning of my birthday, no one at home...but o well, andytoh, i know all of those fundamentals, however I am not 100% sure I have the formal definiton of a limit in my head...and also, somethings that worries me, I understand, but don't like, using the formal limit approach to differentials, I prefer viewing them as infintesimal quantites, helps me understand, rather than just calculate. I find it more intuitive, and makes the chain rule a breeze :D.

 

[mathwonk]

see if you like this approach; there is no need for limits to do derivatives for polynomials, as descartes realized.

the limit point of view says the tangent line is the line which is the limit of secant lines, but algebraically this means it is a limit of lines that meet the curve twice. this should be visible algebraically by saying the intersection point obtained by setting the curve and the tangent line equal to each other should have a double root.

so solve for the slope m that makes the line y = m(x-a)+a^2, (which passes through the point (a,a^2) with slope m), meet the curve y = x^2, twice at the point (a,a^2).

I.e. set y = m(x-a)+a^2 = x^2, and note that x=a is a root. Then solve for the unique m that makes x=a, a double root. see if you get m = 2a.

 

[mathwonk]

dont feel too lonely Gib Z, my mom is 98 today, and is in an assisted living home, and i cannot be there, because I have a doctors appointment myself today.

It is true often that the teen age years are a lonely time, because we have not yet found our community. Look forwaRD TO COLLEGE, and choose it well.

But haVE FUN IN HIGH SCHOOL TOO, YOU MAY NEVER SEE MOST OF THosE PEOPle AGAIN, AS THEY WILL GO DIfFERENT WAys.

 

[Gib Z]

Thanks for the advice, I am going to take College to be the same as University, Australia doesn't really have colleges...

The descartes approach is quite good, though what I meant was I prefer to think of differentails as infinitesimals in my head, and when doing calculations. ie all my textbooks say that it isn't rigourous to treat a derivative as the ratio of 2 infintesimals, and can't use them like normal fractions, but it does it anyway..

 

[Hurkyl]

Well then its nice to know I have a small chance of one day becoming as knowledgeable as yourself :D

Ahh feels a little lonely to be on a physics forum on the morning of my birthday, no one at home...but o well, andytoh, i know all of those fundamentals, however I am not 100% sure I have the formal definiton of a limit in my head...and also, somethings that worries me, I understand, but don't like, using the formal limit approach to differentials, I prefer viewing them as infintesimal quantites, helps me understand, rather than just calculate. I find it more intuitive, and makes the chain rule a breeze :D.

You haven't learned differentials yet. :tongue: (And the chain rule is even more obvious in differential geometry!)

Just remember, thinking in terms of infinitessimals is a crutch -- it might help now to imagine derivatives in terms of naïve infinitessimals, but in the long run you are going to want to turn things around: to use the concepts of calculus to define your intuitive notion of infinitessimal.

Now, a definition is needed for mathematical study, but that doesn't mean the definition is the most important thing. The limit definition of a derivative is just one of its many properties -- we just happened to pick that one as the starting point.

(And that, I think is because it's more "concrete" -- it would probably be difficult for most beginning students to understand what's going on if calculus texts started with an abstract characterization of the derivative)


By the way, have you seen nonstandard analysis? There is a calculus text that teaches with the hyperreals.

 

[Gib Z]

Thanks, I've seen nonstandard analysis in wikipedia, and it acquaints to my simplistic definition needs :S From what I saw in the wikipedia article for differentials, I've learned them...but o well...I can't really envision a form of geometry making the chain rule obvious, buy you obviously know more than me. Btw, Happy 28th yesterday :)

 

[mathwonk]

chain rule: (linear + higher)o(linear + higher)

= (linear)o(linear) +( linear)o(higher)+ (higher)o(inear) +(higher)o(higher)

= linear o linear + higher.

 

[Gib Z]

I think du's canceling out in fractions is simpler, if you don't mind me :) no disrespect guys.

 

[mathwonk]

well that is the genius of leibniz!

 

[interested_learner]

If you are going to be any good at all, you have to keep learning all your life. I am 30 years out of college and I still read constantly. I learned NOTHING that I do at work in college, nothing and I went to about 8 years of college. It was all picked up afterwords. Math is a vast field. You will never catch up. That is the fun!

My only answer to you is to keep running. You are off to a good start. Keep it up.

 

[Tom1992]

i don't agree with the definition of a senior unit of knowledge. i doubt that a senior student knows all that, and definitely doesn't remember all that even if he took all those courses.

i am hoping to get my phd by the time I'm 18, but in order to get there in time, i will have to avoid some of those courses. for example, i have no intention of doing my phd in anything related to pde's, so I'm not going to waste my time taking it. i already feel that i wasted my time reading a textbook in number theory because I'm not going to go into that field either (i never use congruences and mods in any of the subjects that i am studying now). you should only know deeply what you want to specialize in, and just have a mild familiarity of subjects that are very distinct from your specialty.

i found that when you move from one level to the next, you rarely use topics from the previous levels again except very common foundations like derivatives, jacobians, vector spaces, open sets,... for example, after learning implicit differentiation in calculus 1, i never needed to use it again. then after learning curl, div, and grad, stokes' thm, gauss' div thm in calc 2, i never used them again. after studying rings, i never saw rings again. after studying curves and surfaces in R^3, the diff geo moved onto R^n, and i never saw them again nor did i ever used the gaussian curvature, mean curvature, mainardi-codazzi equations, gauss-bonnet thm ever again... my point is that to know everything is a waste of knowledge space. you should only know very well what you want to specialize in and just have a basic familiarity with the other topics.

 

[mathwonk]

well yes and no. i also have worked in a somewhat narrow specialty most of my career and never used certain things, that is true.

but occasionally i have been confronted with problems i could not solve, partly because i ahd not bothered to learn something which i THOUGHT was unrelated, but turned out to be useful after all.

One wants to learn, not as many topics, but as many ideas, as possible. Ideas are useful, but only if we have them in mind.

also sometimes you get bored being specialized and want to have some fun teaching number theory even while doing a career in abelian varieties.

at some point you may enjoy talking collaboratively as well with people in adjoining fields, and it helps to be able to communicate with them, knwing their language. Some one who not only does his own research but also answers questions for others is considered more valuable to a department.

So do specialize deeply, but try to keep up a certain familiarity with things that are generally agreed to be important. what is your special field of interest? maybe we can make some suggestions as to how some of the other fields touch it usefully.

good luck with your PhD.

 

[mathwonk]

heres a little example of stuff that came up and surprized me by its usefulness. I knew little about gauss and stokes, etc, but was interested inm topology and our department was having a seminar to learn de rham cohomology, sheaf version. then i had to teach several variable calculus.

somehow i began to wonder how one would prove thata circle really does wrap around the oprigin at the same time, and i noticed that stokes theorem i.e. greens thm was the right tool.

i.e. the fact that dtheta has integral 2pi around the circle implies the circle does not bound any disc that misses the origin, by greens thm.

i was so excited, i relaized this was actually the key idea behind de rham cohomology, but the people lecturing on the sheaf theory version of DR apparently did not know this.

i went on to include a proof that a sphere has no never zero vector fields by gauss thm in my calc class. later i saw an article on this topic in the American math Monthly, but not as elementary as the version I had discovered myself.

In my thesis i was studying the degree of a mapping of moduli spaces, and the usual technique for that is to use "regular values", but i did not have any at my disposal. It turned out the inverse and implicit function theorems could be applied to the normal bundle of a fiber to substitute for them.

This method was very effective, and had not been used before. It only came to mind because years before i had thought long and hard about those theorems from advanced calculus. I was using them in algebraic geometry but the ideas were the same, once understood deeply.

a beautiful technique in studying theta divisors of abelian varieties is to use the gauss map, as introduced by andreotti, or rather as re - interpreted by griffiths, from andreotti's proof. this is a classical idea used by gauss to measure curvature of surfaces in 3 space, but adapted by andreotti to study the geometry of jacobian varieties of algebraic curves.

fundamentally, the gauss map is an invariant of an embedded hypersurface (or more general surface). once understood, it becomes of interest to calculate it also for a divisor embedded in a complex torus, because like affine space, a complex torus has a trivial tangent bundle!

once you get past the nuts and bolts details of the gauss map and curvature in 3 space, and realize the gauss map is an invariant of all hypersurfaces in manifolds with trivial tangent bundle, you can use it more widely.

even in manifolds with non trivial tangent bundle, the "gauss map" taking a map on points, to its derivative, a map on tangent spaces, is of interest in measuring properties of maps, as developed again by griffiths and carlson as the method of infinitesimal variation of hodge structures, in many settings.

the beautiful and powerful techniques in analytic number theory show that zeroes and poles of complex holomorphic map[pings are intimatel connected to number theoretic proe\perties. see the proof by dircihlet of the reslt on primes in arithmetic progression, which also uses crucially group theory.

the idea behind groups is just that of symmetry, which is why it is useful in many places especially physics.

etc etc...any ideas you understand can be useful. so specialize, but try to understand as deeply as possible those ideas you encounter.

 

[Tom1992]

i also have worked in a somewhat narrow specialty most of my career ...also sometimes you get bored being specialized .

so what does a specialist mathematician do when after several years he gets bored in his specialized area? can an abelian varietist suddenly become a number theorist like you suggested?

 

[Gib Z]

Anyone can suddenly change their area of expertise, but it will take time for them to work up to a certain point. A mathematician can change their area of focus to physics, but it will take some time before the mathematician is able to know as much as a fully fledged physicist?

As to you thinking you are wasting you time with some of the fields, I could not disagree more. It seems you care more about your PhD than the beauty of the mathematics. It does not matter if you ever need it again. Having the knowledge, and even better, an elegant proof is all i desire.

Study mathematics for the beauty of it. It is an art. It does not matter if it has no practicality, no physical usefulness, no human interpretation. If you find a field of mathematics that may be useful, learn it. But take the time to appreciate it.

 

[matt grime]

so what does a specialist mathematician do when after several years he gets bored in his specialized area? can an abelian varietist suddenly become a number theorist like you suggested?

Mathwonk didn't suggest changing speciality. He suggested learning about it and teaching it. Abelian varieties are algebraic geometry, a large field, which has links to algebraic number theory and arithmetic geometry. Fermat's last theorem is a proof that very much requires knowledge from such apparently unrelated-to-the-outsider areas.

If you want a reason to learn more than just one narrow area, then I suggest you read up on one of the Fields Medal winners, Terry Tao.

 

[Tom1992]

Study mathematics for the beauty of it. ...take the time to appreciate it.

perhaps you are right. i still need time to figure out which branch of math i really like and so should do some exploring with various courses to find out is right for me. and as mathwonk and mattgrime suggested, any math course we take may surprisingly turn out useful in whatever area we specialize in later on.

 

[mathwonk]

tom, i do get bored sometimes with my speciality. it does seem possible however to switch to number theory, from abelian varieties, since those subjects are closely related.

also algebraic geometry is so broad, that moving to many other fields, like diff geom, diff top, several complex variables, commutative algebra, number theory, maybe algebraic topology, or even mathematical physics such as string theory or quantum field theory, is quite feasible.

i have friends who have done such a transition. I myself have been an invited speaker at the institute for theoretical physics in trieste, while still a specialist in abelian varieties.

in fact as an algebraic geometer, i have learned and used almost all pure math fields.

it took me a while to choose a specialty as well, as i started in algebra, then algebraic topology, then several complex variables, then algebraic geometry.

i have also taught measure theory and functional analysis, and number theory, but never numerical analysis, lie groups, representation theory, or pde.

but i have used the heat equation in my research. so compared to some, certainly not all number theorists, my training is pretty broad.

 

[mathwonk]

i feel however a certain insecurity at changing specialities, since i am a recognized specialist in my area, and if i change, i start over as a newbie.

but hey, you have to go with what interests you before it is too late, right?

 

[mathwonk]

an algebraic function is also analytic, hence differentiable, hence continuous. thus algebraic geometry is a subspecialty of analytic geometry, analysis, differential topology, and topology. thus one can move backwards into any of those other fields, at least in principle.

 

[andytoh]

i certainly don't know all that stuff, especially not the pde...

i have used the heat equation in my research.

How did you incorportate the heat equation in your research when you don't feel up to par in pde's? Did you crash-read a pde's textbook out of interest and then the heat equation sparkled a light in your algebraic geometry lore? And to do research related to the heat equation, even if it isn't the main focus of your article, don't you have to have great in depth knowledge of the techniques of pdes in its solution methods? I don't get it.

 

[mathwonk]

i examined the heat equation and thought about it and its implications. you don't need a general education in an area to use specific topics from that area.

in particular i read carefully the paper of andreotti and mayer, on period relations for algebraic curves.

after many years, i have an ability to use things in my research that i barely know.

 

[mathwonk]

here is a little example: it is difficult to compute the circumference of a circle, but it is easy to prove that the area of a circle is (1/2) the product of the circumference times the radius.

i am like the guy who cannot compute the circumference, but who can prove that the area is the product of the radius by half the circumference.

 

[andytoh]

i do get bored sometimes with my speciality. it does seem possible however to switch to number theory, from abelian varieties, since those subjects are closely related.

also algebraic geometry is so broad, that moving to many other fields, like diff geom, diff top, several complex variables, commutative algebra, number theory, maybe algebraic topology, or even mathematical physics such as string theory or quantum field theory, is quite feasible.

i have friends who have done such a transition.

How much time off does a specialist usually have to take to read up on the prerequisites in another (related) area to start doing research in this new area? (now here is where being a learning machine really helps)

 

[mathwonk]

I am noty sure, as it does seem daunting, but if I don't have the courage to do it now at 64, I may not get topo many more chances. academically, some universities give a year's support for study in a second disciplne.

the best way is to gradually train in the other area and then at some point, make the jump. Thats what David Mumford the famous fields medalist in algebraic geometry did. he became fascinated with comuters and began using them in his research on understanding and classifying the copmplexity of algebraic surfaces, a lifelong interest of his.

Then at some point he proved a beautiful theorem on moduli spaces, wiht joe harris, using comoputers again, then kleft the subject to go full time into artificail intelligence, and pattern recognition.

Mumford's is a hard act to follow, but maybe still a good example to aspire to.

 

[mathwonk]

here is a little crash course in the heat equation and its use in algebraic geometry.

it has long been known that a cubic curve X in the plane has the structure of a group. this is essentially because any two points determine a line, which meets the curve again in a third point, which determines the sum of the first two.

more topologically, the complex points on a smooth plane curve form a torus, or doughnut with one hole, as you can sort of see by looking at the simplest cubic, a triangle.

Now one can see that a torus can be made into a group as follows: take the complex plane and set equal to zero all points which are linear integral combinations of two vectors with different directions, say 1 and i. I.e. C is a group and {n + mi, for all n,m, in Z} is a subgroup and you take the quotient group C/{n+mi}, which as a group is a product of two circles.

topologically it is also the product of two circles, since it formed by gluing the opposite edges of the parallelogram formed by 0, 1, i, and 1+i, hence a torus. using the weierstrass P function and its derivative, one can embed this torus in the complex plane as a cubic. thus any lattice defines a plane cubic.now riemann or abel or someone back there, showed how to go backwards: i.e. given a complex plane cubic X, it inherits a complex and topological structure from the complex plane C^2, in which it lies, at least once it is compactifed at infinity, and hence it has two independent loops on it, one for each circle, i.e. a "homology basis" in fancy language, called say u and v.

There is also a single holomorphic differential dz, which is well defined on the torus, even though the coordinate z is not, because z is well defined up to translation by an element of the lattice {n + mi} and d of a constanT TRANSLATE IS ZERO.

so we get two complex numbers A and B by path integrating dz around u and around v, and Riemann showed these are independent complex numbers hence give a lattice {nA+mB} in C, which then determines a torus group C/{nA+mB}, which in fact is both analytically and group theoretically isomorphic to the original plane cubic X.

Now where does the heat equation come in? well first riemann showed one could normalize the complex generators A,B of the plane lattice so that one of them is always A = 1, and the other B = t, has positive imaginary part.

then one can write down a Fourier series using t which defines a "theta function". f(z,t). i.e. one first gives a quadratic non homogeneous polynomial with linear coefficient z and quadratic coefficient t, and then exponentiates it, and sums over all integer arguments. (see mumford's tata lectures on theta, where he credits me for this description, but i originally learned it from c.l.siegel.)

this gives one a function of the two variables z and t. we think of t as determining the complex structure of the curve (since from t, one can reconstruct the curve as C/{n+mt}), and z as a coordinate on the curve it self.

for fixed t, i.e. fixing the curve, the theta function is a function of z, hence on the curve, which is not well defined, since it is not doubly periodic, but its zero set is doubly periodic so it defines a well determiend zero locus on the curve which is only one point.

so we have a theta function f(t,z), a function of two complex variables (t,z, where t is thught of as determining a complex torus, and z as a point on the torus.

If in the product CxC with coiordinates (t,z) we mod out by the family of alttices {n+mt}, we get a family of tori, one over each point t, and aglobal theta function whose zeroes determine one point no each torus.

the t line is a moduli space for 1 dimensional tori, and over each number t, we have a copy if the corresponding torus and a distinguished point.

as you may know, this theta function is a characteristic solution of the heat equation, so that pde must contain some useful informaton about curves.

 

[mathwonk]

theta functions and the heat equation, part 2

This really comes into its own in higher dimensions and genera. I.e. Riemann generalized this construction to assign a group to each curve of any genus > 0, as follows: he proved a curve of genus g, i.e. a doughnut with g holes, has g independent holomorphic differentials w1,...,wg, and a homology basis of 2g loops u1,...ug, v1,...vg, and thus determines a g by 2g matrix of path integrals [A, B]. he showed one can again normalize the bases wi and ui,vj, so that the matirx contains a gbyg identity matrix, and another gbyg complex matrix t, with pos. def. imaginary part, i.e. [I, t].

then he wrote down "riemanns theta function" f(t,z) of g complex variables z, and apparently g^2 complex variables t, and if one mods out C^g by the lattice of semi periods i.e. by n + mt, where now n,m are integer g-vectors, one gets a complex g dimensional torus C^g/{nI+mt].

HE ALSO SHOWED THAT THE period matrix t is symmetric so there are really only (g)(g+1)/2 variables t. thus the riemann theta function is a holomorphic function on the product space of points (z,t) in C^g x C^(gxg). Again we can mod out this product to form a family of complex tori, and the theta function determines a family of hyperurfaces, one in each torus. these hypersurfaces are called theta divisors.

Now the inverse problem above is of interest. I.e. given a g diemnsional complex torus, when does it arise as above from a genus g complex curve? This is called the Schottky problem. presumably if so, it should be visible from looking at the theta divisor of the corresponding torus.

Now curves depend on 3g-3 parameters, so In genera 1,2, and 3, essentially all "indecomposable" tori do arise from curves, but in genus 4, curves only have 9 parameters and 4 dimensional complex tori have (4)(5)/2 = 10.

so there is one condition that should specify whether or not a complex 4- torus comes from a genus 4 curve. Riemann shoiwed that tori coming from curves in fact have "singular" theta divisors, i.e. if the torus comes from a curve, there is a kink or node on the theta divisor. This raises the opposite question, do all 4-tori with singular theta divisors come form curves? (those which do are called jacobians, so we are trying to recognize jacobians among all complex tori.)

In his thesis at Columbia, Allan Mayer showed about 1960 that at least locally near a 4 diml jacobian, there is a nbhd where this is true. he did it by observing that jacobians J form a hypersurface of codimension one in the space of all 4 dimensional complex tori, and J is contained in the set N of tori with singular theta divisors, so all he had to do was show that N is also a hyperurface of copdimension one.

But the cauchy data for the heat equation implies that if all theta functions satisfying the ehat equation had singular zero loci, then the theta function would be the identically zero solution of the ebnat equation, and it isnt.

this story goes on. Mayer and Andreotti showed in 1967 that in all genera, jacobians are acomponent of N. then in 1977, Beauville showed that in genus 4, N has exactly one other component, thus completely describing 4 dimensional jacobians geometrically.

More recently Robert Varley and I gave a shorter proof of this corollary of Beauville's more extensive work.

Varley and then i used the ehat equation to show that also in genus 5, N has exactly 2 components, and computed the multiplicity of jacobians J on the correspoing component of N, but did not uniquely specify J there.

if you look at the heat equation you see it equates a second derivative of theta wrt z to a first derivative wrt t. As Andreoti and mayer showed, this gives a geometric relation between tangent directions in the moduli space of tori, with quadratic tangent cones to th theta divisor in the torus itself.

Later Welters gave a completely algebraic proof of this version of the heat equation, so that it makes sense in characteristic p geometry, and Varley and I used that version to generalize a famous result of Mark Green on theta divisors of complex Jacobians, to characteristic p > 2.

thus the heat equation has a completely geometric interpretation that can be used to reason about it, independently of knowing analysis or pde.

 

[andytoh]

Totally amazing! A person who did not read earlier in this thread would think that you are a worldwide guru in both algebraic geometry and pde's.

 

[mathwonk]

a mathematician gets very familiar with his own specialty and the tools that are used in it. Algebraic geometry is unusual in that it concerns the study of a specific class of objects, algebraic varieties, rather than the use of a specific tool.

So algebraic geometers use synthetic gometry, commutative algebra, complex and real analysis, algebraic topology, group theory, differential geometry, and differential equations, to study algebraic varieties. Hence they tend to have a broader acquaintance with other fields than some specialists.

What I outlined above is pretty well known stuff, and many people have a far wider knowledge of these areas than I. But you learn one topic at a time. After a long while it adds up. And the better are the people you read and listen and talk to, the more you benefit.

Many of these famous people are very generous with their time. When I took leave in the early 1980's to go to Harvard to study, David Mumford kindly gave me his prepublicatioon notes for part of his three volume book on theta functions and I lectured on them for an audience including him. That was very enlightening.

He later shared preprints by other mathematicians specializing in theta functions, including Igusa, which contained ideas that came in handy later in some of the work mentioned above.

Over the years my colleague Robert Varley and others have patiently shared their knowledge. When you are in a math department, you have the luxury of learning by asking questions, which is faster than reading, but there is no substitute for lifelong consistent reading of work by experts.

 

[mathwonk]

theta functions and the differential equations, part 3

The Schottky problem of characterizing Jacobians among all complex algebraic tori, also called abelian varieties, was originally an analytic or algebraic question, that of giving actual equations in some appropriate coordinates, such as the matrices t, for the moduli space of abelian varieties that vanish exactly on jacobians.

The problem was given its impetus 100 years ago by Schottky who wrote down some relations which he proved were indeed satisfied by jacobians, but it was hard to show even that these relation were not identically zero, much less that they vanished only on jacobians.

In the 1970's Igusa annunced he could prove in genus 4, that (the closure of) jacobians was the only component of the zero locus of the one genus 4 Schottky relation, and in about 1981 he wrote down the proof. He used a differential equation satisfied by hyperellipic jacobians, to deduce that every possible component of the Schottky locus must pass through the "boundary" locus of degenerate 4 dimensional abelian varieties, i.e. products of 4 elliptic curves (genus one curves).

Then he only had to count the number of components through that locus, which he did by explicitly computing the tangent cone at that locus and showing it was defined by an irreducible polynomial. Since every component of the Schottky locus must contribute at least one component to the normal cone, the irreducibility of the normal cone implied irreducibility of the Schottky locus.

In dimension 5 Varley and I were trying to show the Andreoti Mayer hypersurface N parametrizing 5 dimensional abelian varieties with singular theta divisor, had just 2 components, as Beauville had shown in dimension 4. So we used a modification of Igusa's idea, namely we showed all possible components of N had to pass through the locus of Jacobians having an "even vanishing theta null", and then we were reduced to finding the number of components of N that did pass through that locus.

Unlike Igusa's case we knew there were at least 2 components so we needed a way to count them. Unlike his case also, the normal tangent cone to this locus had an "multiple" component, i.e. one whose algebraic equation had multiplicity greater than one, which we needed to understand, since that can increase the number of normal cone components over the number of actual discriminant locus components.

The classical study by Lefschetz of moduli of singular hypersurfaces with only isolated singular points had been completed by Teissier and Le. Their theory showed that one could compute the multiplicity of the tangent cone at a point of the moduli variety of singular hypersurfaces, i.e. of the "dscriminant locus", using "Milnor numbers", which are a count of the homology cycles in the hyperurface that vanish into the singularity as the hypersurface acquires a pinch or singularity.

The multiplicity of the discriminant locus at a point corresponding to a hypersurface with finitely many singularities equaled the sum of the Milnor numbers at all singularities. We had to generalize this to the case of infinitely many singularities, i.e. a positive dimensional famnily of singular points.

We showed that in the isolated case the sum of the milnor numbers equaled the change in the global euler characteristic of the hypersurface as it acquired a singularity. This version made sense in the infinite singularities case. I.e. we defined the global milnor number to be the change in the euler characteristic, and then showed that we could meaure the multiplicity of the components of the normal cone by this new global milnor number.

Strangely we got multiplicity 3, at the point where we expected only two components to pass. But an interesting phenomenon for theta divisors that is not true for general hypersurfaces, is that on the component containing jacobians, there are in general two ordinary double pointsof the theta divisors. We could show this even by looking at a Jacobian, where there are infinitely many, because we could look in a normal direction and see that only two singularities persisted in a given normal direction under deformation.

To carry out this calculation, we used the geometric interpretation of the heat equation, to study the geometry of the family formed by the union of all the singular loci of all theta divisors, the so called "critical locus".

Still this only handled components that met the one we knew to contain jacobians, so we had to show in fact all divisors on the moduli space of abelian varities must meet. For this we worked out statement by mumford that the Picard group, was isomorphic to Z, and this could be comoputed from the second cohomology group, which in turn was linked to a group cohomology calculation for the "symplectic group" Sp(2g), one of the famous classical matrix groups defined by the standard symplectic form. It also required some homotopy calculations using postnikov towers that one learns about in algebraic topology.

Finally it followed that in fact there were only two global components to the discriminant locus of singular theta divisors in dimension 5, but one of them had "Milnor multiplicity" 2 and the other had multiplicity one. The latter result answered a question attributed to Igusa, by proving that a general abelian variety (of dimension 5) having a vanishing even theta null, only has one of them.

This theory of positive dimensional Milnor numbers was later generalized by Parusinski. you can learn the classical theory, isolated singularity case, from milnor's book on singularities of complex hypersurfaces. Using a different but related technique, involving degeneration to lower dimensions, a sort of geometric induction method, DeBarre later proved the discriminant locus of abelian varieties with singualr theta diviusors has 2 components in all dimensions. I believe he used a beautiful computation of the monodromy group of the Gauss map of a smooth theta divisor.

 

[mathwonk]

geometric heat equation part 4

here is the geometry of the heat equation following andreotti and mayer.


recall the product space C^g x C^(g^2) can be viewed, by projecting on the second coordinate, as a family of complex g spaces, one over each gbyg matrix t.

If we mod out the space C^g over t, by the lattice {nI+mt} where n,m, run over all integer vectors, we get a family of complex tori, one over each matrix point t.

If we restrict this family over only the set H in C^(g^2), of matrices which are symmetric and have positive definite imaginary part, we can write down a convergent Fourier series defining a theta function f(z,t), hence defining a hypersurface f(z,t) = 0, in the 2 vbls (z,t),which can be viewed as a family of hypersurfaces in z, one over each t in H.

now we have a family of g-1 diml hypersurfaces, one over each point t in H. Most of these hypersurfaces are non singular, i.e. smooth, but a codimension one family of t's have a singular hypersurface over them. this codimension one set of t's is called D, the discriminant hypersurface in H, for the family.

the set C of singular points on all theta hypersurfaces, is the common zero locus of the functions f =0, and of the partials of f wrt z. since there are g+1 such functions, the common zero locus does not meet every g diml hypersurface, but does meet a closed subvariety of them.

the closed set D in H, the discriminant locus, consists of those t such that the corresponding theta divisor has at least one singular point, usually only one or two. There is a projection down p:C-->D in H, and we can look at its derivative.

I.e. the total family C of all singular points on all theta divisors, is itself usually smooth, and we can look at the derivative of p as a linear map from the tangent space to C at a singular point (z,t), down to the tangent space to D inside the tangent space to H at t. C and D have the same dimension, one less than H.

now remember we have three nested families upstairs lying over H. we have a family of smooth tori, and in that a family of not always smooth theta divisors, and in that, a family C of singular points on non smooth theta divisors.

look at the projection from these various families down to H. and at the derivative of the projections. since the tori are all smooth, the derivative of the big projection is surjective. since however the theta divisors are not all smooth, the derivative of the projection restricted to the family of theta divisors, fails to be surjective exactly at a singular point (z,t) of a singular theta divisor.

so we could also define the critical locus C, as the points upstairs where the derivative of the restricted projection from the family of theta divisors down to H, has non surjective derivative. now in general the discriminant locus downstairs is a smooth hypersurface D in H, and the image at t in D of the derivative of the restricted projection, is just the tangent space to D at t.

moreover the equation of that tangent hyperplane is the vector of first partials of the theta function wrt t. This is what comes up on one side of the heat equation.

Now at a general singular point upstairs on the singular theta divisor over t in D, the singular point is a double point, at which the first non zero terms of the taylor series are quadratic, and the matrix of this quadric is the symmetric matrix of second partials of the theta function wrt z. that is what is on the other side of the heat equation.

now notice that the tangent space to H consists of symmetric matrices, and on the other hand the tangent space to the vertical space C^g over t in H consists of g diml vectors z1,..zg. Now a quadric tangent cone to a double point of the theta divisor over t, thus is a symmetric matrix of second partials of f wrt z. i.e. a quadratic homogeneous polynomial in the vbls zi.

this quadric cone in C^g, on the other hand can be looked at as a determining a symmetric matrix and hence a tangent vector to H, i.e. as the coordinates of a vector in t space.

the heat equation says these are the same.

i.e. the geometric heat equation says the symmetric gbyg matrix determined by the quadric tangent cone to a double point of the singular theta divisor lying over the point t of D, is the same as the vector in g(g+1)/2 space determining the tangent plane at t to the discriminant locus D in H.


If the theta divisor over t has several singular points, then you get several quadrics and several tangent planes at t in H. The intersection of those tangent planes gives the tangent directions in H of the locus of t's whose theta divisors have as many singular points as does the one at t.

hence if you can compute the dimension of the intersection of those tangent planes at t, you can see how large is the locus of t's having the same number, or same dimension, of singular points as the one at t.

AM showed that near a jacobian period matrix t, the locus of matrices with g-4 dimensional singularities on theta, was 3g-3, exactly the dimension of the set of jacobian matrices. thus near a jacobian, one can recognize another jacobian because the theta divisor has the same size singular locus at as t.

they made the computation by using the heat equation to equate it with the computation of how many quadrics contained a certain canonical model of the curve X defining the jacobian matrix. thus classical geometry in projective space enabled a tangent computation in the moduli space of abelian varieties.


Is not this amazing?

 

[Gib Z]

If I understood more of it, I bet my jaw would be on the ground. From what I *think* you have shown, its pretty amazing.

I'm not sure about which field I want to go into, I was thinking Number Theory but that sounds extremely difficult...You need to know a vast range of mathematics, and I don't have that special gift of seeing a theorem when I see one. For Example, Fermat Would just notice, or intuitively think, about a theorem, and then prove it. Its easier to prove them if you know them. But deriving Number Theorems are extremely difficult, even elementary ones. I don't know something worthwhile when I see it.

 

[mathwonk]

well none of us has to come anywhere near fermat to make a contribution to ma thematics. but we can try to emulate him, and see where it leads us. you can do your own mathematics.this beautiful work of AM just described here, is very unusual in its originality and scope.

i would guess, and it is an informed guess, however that they came up with it by reading riemann and other great 19th century workers.

 

[mathwonk]

If you are interested in this set of posts, try to read it, and work it out on paper. try to confirm the statements made there. if you do not succeed after a while, take it on faith, and proceed to the next statement. this is the way to learn math. if a statement seems hard, try to verify it in dimension one. then try to go up.