Seymour Receives Ostrowski Prize

PAUL D. SEYMOUR of has re- algorithms for all those graph properties which ceived the 2003 Ostrowski Prize recognizing out- are closed under deleting or contracting edges. standing mathematical achievement. The prize car- Recently Seymour and his student Chudnovsky ries a monetary award of 100,000 Swiss francs combined their work with that of Seymour and his (approximately US$80,000) and a fellowship of close collaborators Robertson and Thomas in order 25,000 Swiss francs. The fellowship will be awarded to prove the strong conjecture of to Seymour’s student, . The prize Berge. Berge’s conjecture had stood since 1961 ceremony will take place at the University of Wa- and was one of the most important open problems terloo on September 30 and October 1, 2004. in . The chromatic number of a graph G is the minimum number of colors needed to Citation color the vertices of G so that adjacent vertices have What follows is the citation prepared by the jury different colors. The clique number of G is the of the Ostrowski Prize. largest number of pairwise adjacent vertices of G. Paul Seymour was born in 1950 Those graphs for which the two numbers are equal in England, obtained his D.Phil. from for all induced subgraphs are known as perfect the University of Oxford in 1975, graphs. A hole of a graph is a chordless of and is currently a professor of math- length at least four, and an antihole is the com- ematics at Princeton University. He plement of such a cycle. Berge conjectured that a received the George Pólya Prize in graph is perfect if and only if it contains no odd hole 1983 and the in or antihole. The proof by Chudnovsky, Robertson, 1979 and 1994, the second time Seymour, and Thomas of Berge’s conjecture is a jointly with Neil Robertson and profound contribution to the subject of Robin Thomas. In 1994 he gave a combinatorial . plenary lecture at the International Congress of Mathematicians. About the Prize Paul Seymour has enriched math- The Ostrowski Foundation was created by Alexander ematics with a number of spectacular Ostrowski, for many years a professor at the results. His work is known not only University of Basel. He left his entire estate to the by all discrete mathematicians but foundation and stipulated that the income should also by most theoretical computer Paul D. Seymour provide a prize for outstanding recent achieve- scientists. ments in pure mathematics and the foundations of For instance, Seymour gave a numerical mathematics. The prize is awarded every precise characterization of totally unimodular other year. The prize jury consists of representa- matrices, a result which is one of the deepest in the tives from the universities of Basel, Jerusalem, and theory of . With Robertson and Thomas Waterloo and from the academies of Denmark and he characterized completely the graphs which the Netherlands. For the 2003 prize, the jury cannot be embedded in three-space without two cycles being linked and also solved Pólya’s members are: Joram Lindenstrauss, David Masser, problem from 1913 and the next open Cameron Stewart, Carsten Thomassen, and Robert case (one past the four-color theorem) of Tijdeman. Hadwiger’s conjecture of 1943. Robertson, Sanders, Previous recipients of the Ostrowski Prize are Seymour, and Thomas gave a new and simpler Louis de Branges (1990), (1991), proof of the four-color theorem of Appel and Haken. Miklos Laczkovich (1993), Marina Ratner (1993), Further, in a sequence of papers with Robertson he (1995), Yuri Nesterenko (1997), Gilles proved that, for every infinite collection of finite Pisier (1997), Alexander Beilinson (1999), Helmut graphs, there is always one which can be obtained Hofer (1999), (2001), from another by deleting and contracting edges. (2001), and Richard L. Taylor (2001). Their work provides polynomially bounded —Allyn Jackson

900 NOTICES OF THE AMS VOLUME 51, NUMBER 8