## WAGS @ Colorado State, 15–16 October

The Fall 2016 edition of the Western Algebraic Geometry Symposium (WAGS) will be held at Colorado State University on the weekend of 15–16 October. In addition to an excellent program of talks, there will be a lively poster session.

Speakers are:
Enrico Arbarello, Sapienza Universita di Roma/Stony Brook
Emily Clader, San Francisco State University
Luis Garcia, University of Toronto
Diane Maclagan, University of Warwick
Sandra Di Rocco, KTH
Brooke Ullery, University of Utah

For more information and to register, see the WAGS Fall 2016 site.

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## Our friend the Tate elliptic curve

Rigid analytic spaces are all the rage these days, thanks to the work of Peter Scholze and his collaborators on perfectoid spaces. In this post, I want to briefly describe the example that inspired the whole subject of rigid analytic spaces: the Tate elliptic curve. Tate’s original 1959 notes were not published until 1995. (My thanks to Martin Gallauer for his explanations of the theory.)

Let ${\bf C}_p$ be the completion of the algebraic closure of the p-adic numbers ${\bf Q}_p$. The difficulty in defining analytic spaces over ${\bf C}_p$, by analogy with complex analytic spaces, is that ${\bf C}_p$ is totally disconnected, and so there are too many locally analytic (or even locally constant) functions. Tate became convinced that it should be possible to get around this problem by his discovery of the Tate elliptic curve. Namely, by explicit power series, he argued that some elliptic curves $X$ over ${\bf Q}_p$ could be viewed as a quotient of the affine line minus the origin as an analytic space: ${\bf Q}_p^*/\langle q^{\bf Z}\rangle \cong X({\bf Q}_p).$

Trying to make sense of the formulas led Tate to his definition of rigid analytic spaces. In short, one has to view a rigid analytic space not just as a topological space, but as a space with a Grothendieck topology — that is, a space with a specified class of admissible coverings. So, for example, the closed unit disc $\{ z: |z| \leq 1\}$ acts as though it is connected, because its covering by the two disjoint open subsets $\{ z: |z| < 1\}$ and $\{ z: |z| = 1\}$ is not an admissible covering. (“Affinoids,” playing the role of compact open sets, include closed balls such as $|z|\leq a$ for any real number $a$, but not the open ball $|z|<1$. An admissible covering of an affinoid such as $\{ z: |z| \leq 1\}$ is required to have a refinement by finitely many affinoids.)

Tate’s formulas for the p-adic analytic map $G_m \rightarrow X$, modeled on similar formulas for the Weierstrass $p$-function, are as follows.

Theorem. Let $K$ be a complete field with respect to a non-archimedean absolute value, and let $q \in K^*$ have $0<|q|<1$. Then the following power series define a isomorphism of abelian groups $K^*/q^{\bf Z}\cong X(K)$, for the elliptic curve $X$ below:

$x(w)=\sum_{m\in {\bf Z}}\frac{q^m w}{(1-q^mw)^2} -2s_1$

$y(w)=\sum_{m\in {\bf Z}}\frac{q^{2m} w}{(1-q^mw)^2} +s_1,$

where $s_l=\sum_{m\geq 1}\frac{m^lq^m}{1-q^m}$ for positive integers $l$. The corresponding elliptic curve $X$ in ${\bf P}^2$ is defined in affine coordinates by $y^2+xy=x^3+Bx+C,$ where $B=-5s_3$ and $C=(5s_3+7s_5)/12$. Its $j$-invariant is $j(q)=1/q+744+196884q+\cdots.$ For every element $j\in K$ with $|j|>1$ (corresponding to an elliptic curve over $K$ that does not have potentially good reduction), there is a unique $q\in K$ with $j(q)=j$.

It is worth contemplating why the formulas for $x(w)$ and $y(w)$ make sense, for $w\in K^*$. The series both have poles when $w$ is an integer power of ${q}$, just because these points map to the origin of the elliptic curve, which is at infinity in affine coordinates. More important, these formulas make it formally clear that $x(qw)=x(w)$ and $y(qw)=y(w)$, but the series do not obviously converge; the terms are small for $m \rightarrow \infty$, but they are large for $m\rightarrow -\infty$.

To make sense of the formulas, one has to use the identity of rational functions $\frac{w}{(1-w)^2} = \frac{w^{-1}}{(1-w^{-1})^2}.$ As a result, the series for $x(w)$ (for example) can be written as

$x(w)=\frac{w}{(1-w)^2}+\sum_{m\geq 1}\big(\frac{q^mw}{(1-q^mw)^2}+\frac{q^mw^{-1}}{(1-q^mw^{-1})^2} -2\frac{q^m}{(1-q^m)^2}\big),$

which manifestly converges. One checks from this description that the series $x(w)$ satisfies $x(qw)=x(w)$, as we want.

References:

S. Bosch, U. Güntzer, R. Remmert. Non-Archimedean Analysis. Springer (1984).

B. Conrad. Several approaches to non-Archimedean geometry. P-adic Geometry, 9–63, Amer. Math. Soc. (2008).

W. Lütkebohmert. From Tate’s elliptic curve to abeloid varieties. Pure and Applied Mathematics Quarterly 5 (2009), 1385–1427.

J. Tate. A review of non-Archimedean elliptic functions. Elliptic Curves, Modular Forms, & Fermat’s Last Theorem (Hong Kong, 1993), 162–184. Int. Press (1995).

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## Book review: The Serre–Tate correspondence

For the past month I’ve been punctuating my life by reading the correspondence between Jean-Pierre Serre and John Tate, recently published in two volumes. Anyone interested in the development of number theory and algebraic geometry will find something to enjoy here.

The book was presumably suggested by the success of the Grothendieck–Serre correspondence, published by the Société Mathématique de France in 2001 and in English translation by the American Mathematical Society in 2003. The Grothendieck–Serre correspondence, beyond its outstanding mathematical interest, has the additional personal fascination of Grothendieck’s story. At first a complete outsider to algebraic geometry, he becomes the master builder of the subject in the 1960s, before rejecting mathematics and, by the end, the rest of humanity.

By comparison, Serre and Tate are reasonable men. The attraction of their correspondence lies in the mathematical ideas that they gradually develop, over the years from 1956 to 2009. Some of the key topics are Galois cohomology (essentially created by Serre and Tate), Tate’s notion of rigid analytic spaces, the Tate conjecture on algebraic cycles, Tate’s invention of p-adic Hodge theory, and Serre’s work on the image of Galois representations, for example for elliptic curves.

Serre usually writes in French, and Tate in English; but both writers make occasional use of the other language for the fun of it.

One running theme is Tate’s reluctance to write up or publish some of his best work. Serre encourages Tate and edits Tate’s papers, but sometimes has to concede defeat. Mazur and Serre started to prepare the publication of Tate’s Collected Papers in about 1990, which would include letters and unpublished work; sadly, nothing has appeared. Serre reports that the AMS has revived the project, and concludes: “I cross my fingers.”

A major topic of the correspondence starting in the 1970s is the relation between modular forms and Galois representations. Deligne and Serre showed in 1974 that a modular form of weight 1 determines a Galois representation with image a finite subgroup of PGL(2,C). At that time, however, it was a serious computational problem to give any example at all of a modular form of weight 1 for which the image is an “interesting” subgroup (that is, A4, S4, or A5, not a cyclic or dihedral group). Tate and a group of students found the first example on June 21, 1974. Soon Tate becomes fascinated with the HP25 programmable calculator as a way to experiment in number theory.

Both Serre and Tate are strongly averse to abstract theories unmoored to explicit examples, especially in number theory. This is a very attractive attitude, but it had one unfortunate effect. One of Serre’s best conjectures, saying that odd Galois representations into GL(2) of a finite field come from modular forms, was formulated in letters to Tate in 1973. But for lack of numerical evidence, Serre ended up delaying publication until 1987. The conjecture played a significant role in the lines of ideas leading to Wiles’s proof of Fermat’s last theorem. Serre’s Conjecture was finally proved by Khare and Wintenberger.

Finally, the correspondence has its share of mathematical gossip. One memorable incident is the Fields Medals of 1974. Tate is on the Fields Medal committee, and Serre suggests “Manin-Mumford-Arnold” as not a bad list, with Arnold as the strongest candidate outside number theory and algebraic geometry. In the event, the award went only to two people, Bombieri and Mumford. At least in the case of Arnold, it seems clear (compare this MathOverflow question) that this was a disastrous result of official anti-Semitism in the USSR, with the Soviet representative to the International Mathematical Union, Pontryagin, refusing to allow the medal to go to Arnold.

I hope that some mathematical readers will go on from the Serre–Tate correspondence to Serre’s Collected Papers. Serre took the idea of cohomology from topology into algebraic geometry and then into number theory. He is one of the finest writers of mathematics. I recommend his papers without reservation.

Correspondance Serre–Tate, 2 volumes. Editée par Pierre Colmez et Jean-Pierre Serre. Société Mathématique de France (2015).

Photo was from the Cambridge branch of Cats Protection, but a different cat is now featured.

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## SoCalAGS @ UCLA, 5 December 2015

Don’t displease Brian Eno’s cat. Come to the four excellent talks at December’s Southern California Algebraic Geometry Seminar at UCLA.

Talks are in Mathematical Sciences Building 6627.

Speakers are:
Jim Bryan, University of British Columbia
Dragos Oprea, University of California, San Diego
Giulia Sacca, SUNY Stony Brook
Xinwen Zhu, Caltech

For information on parking, please register.

Photo is a still from The British Guide to Showing Off via Hero Culte.

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## WAGS @ UW, 17–18 October 2015

The Fall 2015 Western Algebraic Geometry Symposium will be held at the University of Washington, Seattle, on the weekend of 17–18 October in Savery Hall 260 (on the Quad — not the math department building).

Speakers are:
Valery Alexeev, Georgia
Aravind Asok, USC
Brian Osserman, Davis
Alena Pirutka, NYU
Yiwei She, Columbia
Rekha Thomas, Washington
Nikolaos Tziolas, Cyprus & Princeton

Photo from imagur. Todd Marinovich: “I just saw purple. That’s all. No numbers, no faces, just purple.”

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## New paper: Rationality does not specialize among terminal varieties

I’ve posted a new paper on the arXiv. A limit of rational varieties need not be rational, even if all varieties in the family are projective and have at most terminal singularities. This shows that a result of de Fernex and Fusi’s does not extend to higher dimensions.

Photo: Susie the cat in Westwood.

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## Drama by math department standards

He eschews job offers from prestigious East Coast institutions in favor of a relaxed, no-drama department in a place where he can enjoy the weather. — NYT on Terry Tao

Number theorist questions algebraist’s notation.
French mathematician writes paper in French.
Seminar speaker has not stated theorem with 5 minutes remaining.
Geometer attends statistics talk.
Blackboard erasers disappear. Reappear after cleaning.
Colloquium speaker is a vegetarian.
Probabilist volunteers to teach analysis.

— Guest post