# Robert F. Coleman 1954-2014

I am very sad to report that my Ph.D. advisor, Robert Coleman, died last night in his sleep at the age of 59.  His loving wife Tessa called me this afternoon with the heartbreaking news.  Robert was a startlingly original and creative mathematician who has had a profound influence on modern number theory and arithmetic geometry.  He was an inspiration to me and many others and will be dearly missed.

Robert and Bishop in Paris

When I was a graduate student at Berkeley, Robert hosted an invitation-only wine and cheese gathering in his office every Friday afternoon code-named “Potatoes”.  Among the regular attendees were Loïc Merel and Kevin Buzzard, who were postdocs at the time.  It was a wonderful tradition.  In the summer of 1997, while I was still a graduate student, Robert invited me to accompany him for three weeks in Paris to a workshop  on p-adic Cohomology at the Institut Henri Poincare.  That was the first time I met luminaries like Faltings, Fontaine, and Mazur.  Since the workshop was (a) totally in French and (b) on a topic I knew almost nothing about, I was in completely over my head.  But I fell in love with Paris (which I’ve since returned to many times) and my best memories from that trip are of dining with Robert and seeing the city with him.

The trip also taught me to appreciate the significant challenges which Robert, who had Multiple Sclerosis, bravely faced every day.  I remember helping Robert check into his hotel room near the Luxembourg Gardens, only to find out that his wheelchair did not fit in the elevator.  We had to find another hotel room for him, which was not so easy given the level of our French!  Curbs were a constant challenge for Robert, as finding on- and off- ramps for wheelchairs in Paris was like trying to get a vegan meal in rural Arkansas.

# Lucas sequences and Chebyshev polynomials

This is a sequel to last week’s blog post “Two applications of finite fields to computational number theory“.  The main reason I decided to write a follow-up is that I’ve learned a lot about Concluding Observations #1 and #6 from that post during the last week.   In Observation #1, I mentioned without further comment a recursive procedure for computing square roots modulo a prime; it turns out that this procedure is essentially equivalent to Cipolla’s algorithm, but was discovered independently by Lehmer (who it seems did not know about Cipolla’s work).  I learned this from the wonderful book “Édouard Lucas and Primality Testing” by Hugh C. Williams, which I highly recommend to anyone interested in the history of mathematics.  In explaining the connection between the algorithms of Cipolla and Lehmer, I’ll make a digression into the general theory of Lucas sequences, which I had some vague knowledge of before but which I’ve learned a lot about in the last week from reading Williams’ book.  In Observation #6, I asked if there was a conceptual explanation for the fact that the Chebyshev polynomial $x^2 - 2$ shows up in the Lucas-Lehmer test; Greg Kuperberg sent me just such an explanation and I will expand on his comments below. Continue reading

# Two applications of finite fields to computational number theory

In this post I’d like to discuss how finite fields (and more generally finite rings) can be used to study two fundamental computational problems in number theory: computing square roots modulo a prime and proving primality for Mersenne numbers.  The corresponding algorithms (Cipolla’s algorithm and the Lucas-Lehmer test) are often omitted from undergraduate-level number theory courses because they appear, at first glance, to be relatively sophisticated.  This is a shame because these algorithms are really marvelous — not to mention useful — and I think they’re more accessible than most people realize. I’ll attempt to describe these two algorithms assuming only a standard first-semester course in elementary number theory and familiarity with complex numbers, without assuming any background in abstract algebra.  These algorithms could also be used in a first-semester abstract algebra course to help motivate the practical utility of concepts like rings and fields.

Anecdotal Prelude

In researching this blog post on Wikipedia, I learned a couple of interesting historical tidbits that I’d like to share before getting on with the math.

The French mathematician Edouard Lucas (of Lucas sequence fame) showed in 1876 that the 39-digit Mersenne number $2^{127}-1$ is prime.  This stood for 75 years as the largest known prime.  Lucas also invented (or at least popularized) the Tower of Hanoi puzzle and published the first description of the Dots and Boxes game.  He died in quite an unusual way: apparently a waiter at a banquet dropped what he was carrying and a piece of broken plate cut Lucas on the cheek. Lucas developed a severe skin inflammation and died a few days later at age 47.

Derrick Henry Lehmer was a long-time professor of mathematics at U.C. Berkeley.  In 1950, Lehmer was one of 31 University of California faculty members fired for refusing to sign a loyalty oath during the McCarthy era.  In 1952, the California Supreme Court declared the oath unconstitutional, and Lehmer returned to Berkeley shortly thereafter.  Lehmer also built a number of fascinating mechanical sieve machines designed to factor large numbers. Continue reading

# Primitive roots, discrete logarithms, and p-adic numbers

This morning I attended Martin Hellman’s stimulating keynote address at the 2013 Georgia Tech Cyber Security Summit. Martin Hellman is the co-inventor, with Whitfield Diffie, of the Diffie-Hellman Key Exchange Protocol, which began the (public) public-key cryptography revolution.  Among the interesting things I learned during Martin Hellman’s talk are:

1. Hellman feels that Ralph Merkle deserves equal credit for inventing public-key cryptography and refers to his own invention as the Diffie-Hellman-Merkle key exchange protocol.  (Merkle was the director of the Georgia Tech Information Security Center from 2003-2006.)

2. Hellman came up with the famous “double padlock” thought experiment after the invention of the Diffie-Hellman-Merkle key exchange protocol, as a way to explain it to others.  The mathematics came first.  (I had always wondered about this.)

3. Most interestingly, Hellman said that he got the idea to use modular exponentiation/discrete logarithms as a “one-way function” from the engineer and mathematician John Gill (who I never heard of before this morning).  John Gill’s other suggestion was to use multiplication/factoring, which forms the basis of RSA!  It’s all the more amazing that I’ve never heard of John Gill because he earned his bachelor’s degree in Applied Mathematics from Georgia Tech (where I now teach) and his Ph.D. in Mathematics from U.C. Berkeley (where I got my Ph.D.)!  Hellman also recounted a conversation in which Gill (who is African-American) mentioned having encountered very little racial intolerance during his undergraduate studies in the 1960’s — apparently Georgia Tech was (relatively speaking) an oasis of tolerance among southern universities during that time.

Now on to the mathematical part of this post, which is an unusual proof of the existence of primitive roots modulo primes which I came up with recently while preparing a lecture for my course on Number Theory and Cryptography.  The proof is much less elementary than every other proof I’ve seen, but I would argue that it nevertheless has some merit.   Continue reading

# Riemann-Roch for Graphs and Applications

I plan to write several posts related to the Riemann-Roch Theorem for Graphs, which was published several years ago in this paper written jointly with Serguei Norine.  In this post I want to explain the statement of the theorem, give some anecdotal background, and mention a few applications which have been discovered in recent years.

The Riemann-Roch Theorem

The (classical) Riemann-Roch Theorem is a very useful result about analytic functions on compact one-dimensional complex manifolds (also known as Riemann surfaces).  Given a set of constraints on the orders of zeros and poles, the Riemann-Roch Theorem computes the dimension of the space of analytic functions satisfying those constraints.  More precisely, if $D$ denotes the set of constraints and $r(D)$ is the dimension of the space of analytic functions satisfying those constraints, then the Riemann-Roch theorem asserts that

$r(D) - r(K-D) = {\rm deg}(D) + 1 - g$

where $g$ is the genus (“number of holes”) of the Riemann surface $X$, ${\rm deg}(D)$ is the total number of constraints, and $K$ is the “canonical divisor” on $X$.  See the Wikipedia page for much more information.