Paul Seymour (mathematician)

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Paul Seymour
PaulSeymour2010.jpg
Born (1950-07-26) 26 July 1950 (age 70)
Plymouth, Devon, England
NationalityBritish
Alma mater University of Oxford (BA, PhD)
Awards Sloan Fellowship (1983)
Ostrowski Prize (2003)
George Pólya Prize (1983, 2004)
Fulkerson Prize (1979, 1994, 2006, 2009)
Scientific career
Institutions Princeton University
Bellcore
University of Waterloo
Rutgers University
Ohio State University
Doctoral advisor Aubrey William Ingleton
Doctoral students Maria Chudnovsky
Sang-il Oum

Paul D. Seymour (born 26 July 1950) is a British mathematician known for his work in discrete mathematics, especially graph theory. He (with others) was responsible for important progress on regular matroids and totally unimodular matrices, the four colour theorem, linkless embeddings, graph minors and structure, the perfect graph conjecture, the Hadwiger conjecture, claw-free graphs, χ-boundedness, and the Erdős–Hajnal conjecture. Many of his recent papers are available from his website. [1]

Contents

Seymour is currently the Albert Baldwin Dod Professor of Mathematics at Princeton University. [2] He won a Sloan Fellowship in 1983, and the Ostrowski Prize in 2004; and (sometimes with others) won the Fulkerson Prize in 1979, 1994, 2006 and 2009, and the Pólya Prize in 1983 and 2004. He received an honorary doctorate from the University of Waterloo in 2008 and one from the Technical University of Denmark in 2013.

Early life

Seymour was born in Plymouth, Devon, England. He was a day student at Plymouth College, and then studied at Exeter College, Oxford, gaining a BA degree in 1971, and D.Phil in 1975.

Career

From 1974 to 1976 he was a college research fellow at University College of Swansea, and then returned to Oxford for 1976–1980 as a Junior Research Fellow at Merton College, Oxford, with the year 1978–79 at University of Waterloo. He became an associate and then a full professor at Ohio State University, Columbus, Ohio, between 1980 and 1983, where he began research with Neil Robertson, a fruitful collaboration that continued for many years. From 1983 until 1996, he was at Bellcore (Bell Communications Research), Morristown, New Jersey (now Telcordia Technologies). He was also an adjunct professor at Rutgers University from 1984 to 1987 and at the University of Waterloo from 1988 to 1993. He became professor at Princeton University in 1996. He is Editor-in-Chief (jointly with Carsten Thomassen) for the Journal of Graph Theory , and an editor for Combinatorica and the Journal of Combinatorial Theory, Series B .

Paul Seymour in 2007
(photo from MFO) Paul Seymour.jpeg
Paul Seymour in 2007
(photo from MFO)

Personal life

He married Shelley MacDonald of Ottawa in 1979, and they have two children, Amy and Emily. The couple separated amicably in 2007. His brother Leonard W. Seymour is Professor of gene therapy at Oxford University. [3]

Major contributions

Combinatorics in Oxford in the 1970s was dominated by matroid theory, due to the influence of Dominic Welsh and Aubrey William Ingleton. Much of Seymour's early work, up to about 1980, was on matroid theory, and included three important matroid results: his D.Phil. thesis on matroids with the max-flow min-cut property (for which he won his first Fulkerson prize); a characterisation by excluded minors of the matroids representable over the three-element field; and a theorem that all regular matroids consist of graphic and cographic matroids pieced together in a simple way (which won his first Pólya prize). There were several other significant papers from this period: a paper with Welsh on the critical probabilities for bond percolation on the square lattice; a paper in which the cycle double cover conjecture was introduced; a paper on edge-multicolouring of cubic graphs, which foreshadows the matching lattice theorem of László Lovász; a paper proving that all bridgeless graphs admit nowhere-zero 6-flows, a step towards Tutte's nowhere-zero 5-flow conjecture; and a paper solving the two-paths problem, which was the engine behind much of Seymour's future work.

In 1980 he moved to Ohio State University, and began work with Neil Robertson. This led eventually to Seymour's most important accomplishment, the so-called "Graph Minors Project", a series of 23 papers (joint with Robertson), published over the next thirty years, with several significant results: the graph minors structure theorem, that for any fixed graph, all graphs that do not contain it as a minor can be built from graphs that are essentially of bounded genus by piecing them together at small cutsets in a tree structure; a proof of a conjecture of Wagner that in any infinite set of graphs, one of them is a minor of another (and consequently that any property of graphs that can be characterised by excluded minors can be characterised by a finite list of excluded minors); a proof of a similar conjecture of Nash-Williams that in any infinite set of graphs, one of them can be immersed in another; and polynomial-time algorithms to test if a graph contains a fixed graph as a minor, and to solve the k vertex-disjoint paths problem for all fixed k.

In about 1990 Robin Thomas began to work with Robertson and Seymour. Their collaboration resulted in several important joint papers over the next ten years: a proof of a conjecture of Sachs, characterising by excluded minors the graphs that admit linkless embeddings in 3-space; a proof that every graph that is not five-colourable has a six-vertex complete graph as a minor (the four-colour theorem is assumed to obtain this result, which is a case of Hadwiger's conjecture); with Dan Sanders, a new, simplified, computer based proof of the four-colour theorem; and a description of the bipartite graphs that admit Pfaffian orientations. In the same period, Seymour and Thomas also published several significant results: (with Noga Alon) a separator theorem for graphs with an excluded minor, extending the planar separator theorem of Richard Lipton and Robert Tarjan; a paper characterizing treewidth in terms of brambles; and a polynomial-time algorithm to compute the branch-width of planar graphs.

In 2000 Robertson, Seymour, and Thomas were supported by the American Institute of Mathematics to work on the strong perfect graph conjecture, a famous open question that had been raised by Claude Berge in the early 1960s. Seymour's student Maria Chudnovsky joined them in 2001, and in 2002 the four jointly proved the conjecture. Seymour continued to work with Chudnovsky, and obtained several more results about induced subgraphs, in particular (with Cornuéjols, Liu, Vuskovic) a polynomial-time algorithm to test whether a graph is perfect, and a general description of all claw-free graphs. Other important results in this period include: (with Seymour's student Sang-il Oum) fixed-parameter tractable algorithms to approximate the clique-width of graphs (within an exponential bound) and the branch-width of matroids (within a linear bound); and (with Chudnovsky) a proof that the roots of the independence polynomial of every claw-free graph are real.

In the 2010s Seymour worked mainly on χ-boundedness and the Erdős–Hajnal conjecture. In a series of papers with Alex Scott and partly with Chudnovsky, they proved two conjectures of András Gyárfás, that every graph with bounded clique number and sufficiently large chromatic number has an induced cycle of odd length at least five, and has an induced cycle of length at least any specified number. The series culminated in a paper of Scott and Seymour proving that for every fixed k, every graph with sufficiently large chromatic number contains either a large complete subgraph or induced cycles of all lengths modulo k, which leads to the resolutions of two conjectures of Gil Kalai and Roy Meshulam connecting the chromatic number of a graph with the homology of its independence complex. There was also a polynomial-time algorithm (with Chudnovsky, Scott, and Chudnovsky and Seymour's student Sophie Spirkl) to test whether a graph contains an induced cycle with length more than three and odd. Most recently, the four jointly resolved the 5-cycle case of the Erdős–Hajnal conjecture, which says that every graph without an induced copy of the 5-cycle contains an independent set or a clique of polynomial size.

See also

Related Research Articles

In graph theory, the Robertson–Seymour theorem states that the undirected graphs, partially ordered by the graph minor relationship, form a well-quasi-ordering. Equivalently, every family of graphs that is closed under minors can be defined by a finite set of forbidden minors, in the same way that Wagner's theorem characterizes the planar graphs as being the graphs that do not have the complete graph K5 or the complete bipartite graph K3,3 as minors.

In graph theory, an undirected graph H is called a minor of the graph G if H can be formed from G by deleting edges and vertices and by contracting edges.

Perfect graph

In graph theory, a perfect graph is a graph in which the chromatic number of every induced subgraph equals the order of the largest clique of that subgraph. Equivalently stated in symbolic terms an arbitrary graph is perfect if and only if for all we have .

In graph theory, the perfect graph theorem of László Lovász states that an undirected graph is perfect if and only if its complement graph is also perfect. This result had been conjectured by Berge, and it is sometimes called the weak perfect graph theorem to distinguish it from the strong perfect graph theorem characterizing perfect graphs by their forbidden induced subgraphs.

In graph theory, the strong perfect graph theorem is a forbidden graph characterization of the perfect graphs as being exactly the graphs that have neither odd holes nor odd antiholes. It was conjectured by Claude Berge in 1961. A proof by Maria Chudnovsky, Neil Robertson, Paul Seymour, and Robin Thomas was announced in 2002 and published by them in 2006.

In topological graph theory, a mathematical discipline, a linkless embedding of an undirected graph is an embedding of the graph into three-dimensional Euclidean space in such a way that no two cycles of the graph are linked. A flat embedding is an embedding with the property that every cycle is the boundary of a topological disk whose interior is disjoint from the graph. A linklessly embeddable graph is a graph that has a linkless or flat embedding; these graphs form a three-dimensional analogue of the planar graphs. Complementarily, an intrinsically linked graph is a graph that does not have a linkless embedding.

The Fulkerson Prize for outstanding papers in the area of discrete mathematics is sponsored jointly by the Mathematical Optimization Society (MOS) and the American Mathematical Society (AMS). Up to three awards of $1,500 each are presented at each (triennial) International Symposium of the MOS. Originally, the prizes were paid out of a memorial fund administered by the AMS that was established by friends of the late Delbert Ray Fulkerson to encourage mathematical excellence in the fields of research exemplified by his work. The prizes are now funded by an endowment administered by MPS.

Hadwiger conjecture (graph theory)

In graph theory, the Hadwiger conjecture states that if G is loopless and has no minor then its chromatic number satisfies . It is known to be true for . The conjecture is a generalization of the four-color theorem and is considered to be one of the most important and challenging open problems in the field.

Hadwiger number

In graph theory, the Hadwiger number of an undirected graph G is the size of the largest complete graph that can be obtained by contracting edges of G. Equivalently, the Hadwiger number h(G) is the largest number k for which the complete graph Kk is a minor of G, a smaller graph obtained from G by edge contractions and vertex and edge deletions. The Hadwiger number is also known as the contraction clique number of G or the homomorphism degree of G. It is named after Hugo Hadwiger, who introduced it in 1943 in conjunction with the Hadwiger conjecture, which states that the Hadwiger number is always at least as large as the chromatic number of G.

Claw-free graph

In graph theory, an area of mathematics, a claw-free graph is a graph that does not have a claw as an induced subgraph.

In graph theory, a branch of mathematics, many important families of graphs can be described by a finite set of individual graphs that do not belong to the family and further exclude all graphs from the family which contain any of these forbidden graphs as (induced) subgraph or minor. A prototypical example of this phenomenon is Kuratowski's theorem, which states that a graph is planar if and only if it does not contain either of two forbidden graphs, the complete graph K5 and the complete bipartite graph K3,3. For Kuratowski's theorem, the notion of containment is that of graph homeomorphism, in which a subdivision of one graph appears as a subgraph of the other. Thus, every graph either has a planar drawing or it has a subdivision of one of these two graphs as a subgraph.

Branch-decomposition

In graph theory, a branch-decomposition of an undirected graph G is a hierarchical clustering of the edges of G, represented by an unrooted binary tree T with the edges of G as its leaves. Removing any edge from T partitions the edges of G into two subgraphs, and the width of the decomposition is the maximum number of shared vertices of any pair of subgraphs formed in this way. The branchwidth of G is the minimum width of any branch-decomposition of G.

Clique-sum

In graph theory, a branch of mathematics, a clique-sum is a way of combining two graphs by gluing them together at a clique, analogous to the connected sum operation in topology. If two graphs G and H each contain cliques of equal size, the clique-sum of G and H is formed from their disjoint union by identifying pairs of vertices in these two cliques to form a single shared clique, and then possibly deleting some of the clique edges. A k-clique-sum is a clique-sum in which both cliques have at most k vertices. One may also form clique-sums and k-clique-sums of more than two graphs, by repeated application of the two-graph clique-sum operation.

András Hajnal was a professor of mathematics at Rutgers University and a member of the Hungarian Academy of Sciences known for his work in set theory and combinatorics.

In graph theory, an area of mathematics, an equitable coloring is an assignment of colors to the vertices of an undirected graph, in such a way that

Combinatorica is an international journal of mathematics, publishing papers in the fields of combinatorics and computer science. It started in 1981, with László Babai and László Lovász as the editors-in-chief with Paul Erdős as honorary editor-in-chief. The current editors-in-chief are László Babai, László Lovász, and Alexander Schrijver. The advisory board consists of Ronald Graham, András Hajnal, Gyula O. H. Katona, Miklós Simonovits, and Vera Sós. It is published by the János Bolyai Mathematical Society and Springer Verlag.

Bull graph

In the mathematical field of graph theory, the bull graph is a planar undirected graph with 5 vertices and 5 edges, in the form of a triangle with two disjoint pendant edges.

Apex graph

In graph theory, a branch of mathematics, an apex graph is a graph that can be made planar by the removal of a single vertex. The deleted vertex is called an apex of the graph. It is an apex, not the apex because an apex graph may have more than one apex; for example, in the minimal nonplanar graphs K5 or K3,3, every vertex is an apex. The apex graphs include graphs that are themselves planar, in which case again every vertex is an apex. The null graph is also counted as an apex graph even though it has no vertex to remove.

In the mathematical theory of matroids, a minor of a matroid M is another matroid N that is obtained from M by a sequence of restriction and contraction operations. Matroid minors are closely related to graph minors, and the restriction and contraction operations by which they are formed correspond to edge deletion and edge contraction operations in graphs. The theory of matroid minors leads to structural decompositions of matroids, and characterizations of matroid families by forbidden minors, analogous to the corresponding theory in graphs.

In graph theory, a branch of mathematics, the Erdős–Hajnal conjecture states that families of graphs defined by forbidden induced subgraphs have either large cliques or large independent sets. It is named for Paul Erdős and András Hajnal.

References

  1. Seymour, Paul. "Online Papers" . Retrieved 26 April 2013.
  2. https://dof.princeton.edu/about/faculty/professorships
  3. http://news.bbc.co.uk/1/hi/health/6251303.stm