A problem

Since I have been saying rather a lot about association schemes and coherent configurations lately, I thought I would mention an open problem. This is probably one for the experts, and I guess it has been ignored because of the problems of terminology I mentioned in the last post (and also because it appeared in a conference proceedings).

So to recall: a coherent configuration is, in essence, the combinatorial structure arising from the partition of the set of ordered pairs from X into orbits of a permutation group on X. In order of increasing specialisation, we have the following kinds of structure:

  • coherent configurations;
  • homogeneous configurations (where the diagonal is a single relation rather than a union of relations: this would arise in the case that the group is transitive on X;
  • commutative configurations (where the relation matrices commute; this would arise in the case where the group action is multiplicity-free);
  • symmetric configurations (where the relations are all symmetric; this would arise in the case where the group is generously transitive, that is, any pair of points of X can be interchanged by an element of G).

I will use the term association scheme for “symmetric coherent configuration”. If you prefer a different convention, you may think the problem I pose is less interesting; but the interest of a problem should not depend on the language used to describe it!

One of the features of coherent configurations which is crucial for their use in the graph isomorphism problem is that the coherent configurations on a given finite set form a lattice. The join operation in the lattice is just the join in the lattice of set partitions. The meet is defined because there is at least one coherent configuration finer than two given configurations (namely the trivial configuration where all relations are singletons), and the meet of the two is then just the join of all the configurations finer than both.

A permutation group G on X is

  • primitive if it is transitive and preserves no non-trivial partition of X;
  • basic if it is primitive and preserves no Hamming structure (Cartesian power structure) on X;
  • 2-homogeneous if it acts transitively on the set of unordered pairs of distinct elements of X;
  • 2-transitive if it acts transitively on the set of ordered pairs of distinct elements of X;
  • generously transitive if any two distinct points of X can be exchanged by an element of G.

These are all standard in permutation group theory, but we need to add a few more, defined in terms of association schemes. The permutation group is

  • stratifiable if the configuration formed by the symmetrised orbits of G on ordered pairs (taking the union of converse pairs of orbits) is an association scheme;
  • AS-friendly if there is a unique minimal G-invariant association scheme on X;
  • AS-free if the only G-invariant association scheme is the one with just two classes.

These concepts are related by the following implications:

implications

Moreover, none of these implications reverses, and no further implications hold except for those obtained from these by the transitive law.

This theorem, and examples to prove the claims, are in my paper “Coherent configurations, association schemes, and permutation groups”, in Groups, Combinatorics and Geometry, World Scientific, Singapore, 2003.

The problem concerns AS-free groups. These are primitive, and basic (since otherwise they would preserve a Hamming association scheme), and so, by the O’Nan–Scott theorem, they are affine, diagonal or almost simple. The affine groups must be 2-homogeneous. Almost simple examples have been constructed computationally by Faradzev, Klin and Muzichuk; but they are few and far between, and there seems to be no pattern to their occurrence.

This leaves diagonal groups. Now a diagonal group whose socle has two simple factors preserves the conjugacy class scheme of the simple group; and one with three simple factors preserves the strongly regular Latin square graph associated with the Cayley table of the simple group. For four or more factors, the answer is unknown.

Problem: Are primitive diagonal groups with four or more factors in their socles AS-free or not?

The smallest possible degree of such a group would be 216000, so probably computation is not feasible …

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Symmetry vs Regularity

This is the title of a conference to be held next year (2018) from 1 to 7 July, in Pilsen, Bohemia, Czech Republic. The subtitle is “The first 50 years since Weisfeiler-Leman stabilization”, and the webpage is here.

If you have heard Laci Babai talking about his quasi-polynomial time algorithm for graph isomorphism, you might appreciate the significance of this; but if not, it might look a bit specialised. I want to say a bit about the subject matter of the conference, and persuade you that it is a central area of algebraic combinatorics which connects with many other areas of mathematics.

The area grew from three input streams, which resulted in three different names and some continuing confusion. I’ll describe them one at a time, and say a bit about the subsequent history.

Association schemes in design of experiments

If you design an experiment for comparing a number of factors, you have to look ahead to how useful information will be extracted from the experimental results. In all but the simplest cases, this involves finding a generalized (von Neumann) inverse of a potentially rather large concurrence matrix.

R. C. Bose and his school invented the concept of partially balanced designs to facilitate this, replacing inverting a large matrix to inverting a much smaller matrix. This tylically involves having a structure known as an association scheme on the set of treatments, so that the number of blocks in which a pair of treatments occur depends only on the associate class containing the pair of treatments.

So let us stop and define this object. It is a partition of the set of all ordered pairs from the set X of treatments into relations R0, … Rs, with the properties

  • each Ri is symmetric;
  • the diagonal {(x,x):xX} is a single relation, say R0;
  • for all i,j,k, and all pairs (x,y) in Rk, the number of zX such that (x,z)∈Ri and (z,y)∈Rj depends only on i,j,k, and not on (x,y); these numebrs are denoted by pijk.

The third condition looks complicated, but it simply says that the linear span over the real numbers of the matrices of the relations is closed under multiplication, and so forms a commutative algebra, which is called the Bose–Mesner algebra of the association scheme. From the point of view of the statistician, inverting an n×n matrix is replaced by inverting an element of this algebra (represented by an (s+1)×(s+1) matrix in the regular representation, usually very much smaller.

The smallest non-trivial case is s=2, where the relations R1 and R2 are the edge sets of a complementary pair of strongly regular graphs.

The Bose–Mesner algebra thus has two bases: the first consists of the relation matrices; the second consists of the orthogonal projections onto the eigenspaces. The change of basis matrices between these two are now denoted by P and Q.

Cellular algebras in graph isomorphism

The second stream flowed from the former Soviet Union, in particular from the work of Weisfeiler and Leman on graph isomorphism. They defined a class of combinatorial structures, which now are called coherent configurations for reasons we will see shortly; these generalise association schemes in two ways. Again we partition the set of ordered pairs into relations, but we weaken the first two conditions as follows:

  • the transpose of any relation R in the configuration is another relation (not necessarily the same as R);
  • the diagonal is a union of some of the relations in the configuration (not necessarily just one).

The third condition remains the same; so the relation matrices once again span an algebra, though this is no longer necessarily commutative. This algebra was called a cellular algebra, but this term has been hijacked to have a completely different meaning. Taking terminology from the third stream, we call the set-up a coherent configuration, and the algebra a coherent algebra.

How is this relevant to graph isomorphism? We begin by remarking that, if we could find (generators for) the automorphism group of a graph, we could solve graph isomorphism. For, given two graphs, without loss of generality they are both connected; now the two graphs are isomorphic if and only if their disjoint union admits an automorphism interchanging the two components.

Now given a graph, we think of it as a partition of the set of ordered pairs into three parts: the diagonal; the edges of the graph; and the non-edges of the graph.

Now, given any partition of the pairs, we can refine it to a partition which is an association scheme, and can be computed effectively from the graph. First, for each triple i,j,k of indices of relations, and each pair (x,y) in Rk, we count the number of points z such that (x,z)∈Ri and (z,y)∈Rj; then we split Rk into parts corresponding to the different values of this parameter. We repeat this process until it stabilises; when this happens, the values pijk are well-defined, and we have a coherent configuration.

Moreover, this configuration is invariant under the automorphism group of the graph. If it is the trivial configuration, in which all parts are singletons, then the automorphism group is the trivial group. Otherwise, we can try to show that there is a vertex whose stabiliser is trivial, by splitting one of the diagonal relations into a pair (x,x) and all the rest, and repeat the refinement. I will not give further details. But the upshot is that we would like to find a reasonably small set of vertices whose pointwise stabiliser is trivial, and then work back to find appropriate automorphisms.

Coherent configurations and permutation groups

The third strand is the oldest, going back to Schur, who invented the method of Schur rings in his proof that a primitive permutation group containing a regular cyclic subgroup of composite order must be doubly transitive. The method was developed by Schur’s student Wielandt, and included as the fourth of five chapters of his influential book on permutation groups. The translation, by R. Bercov, was my own introduction to the subject in 1968.

Wielandt’s book in fact segues from Schur rings into representation theory, and it is clear that he had thought deeply about this transition. But he didn’t publish much, and the idea was taken up by Donald Higman in the late 1960s. Higman defined a coherent configuration, as we have done above, and observed that, if G is any permutation group, then the orbits of G on ordered pairs form a coherent configuration.

If this configuration is an association scheme, then the group will act on the eigenspaces of the Bose–Mesner algebra, and indeed will be irreducible on each eigenspace. Higman saw the general theory as a way of decomposing permutation representations into irreducible components. Things are more difficult because of the non-commutativity, but the closure under transposition has the result that the algebra is semisimple, so is isomorphic to a direct sum of matrix algebras. In principle, the dimensions of these matrix algebras and their multiplicities in the Bose–Mesner algebra can be computed from the parameters of the configuration. In the group case, this algebra is precisely the centraliser of the permutation matrices making up the group; so on interchanging degrees and multiplicities, we get information about the representation of the group and its decomposition.

Higman spent the year 1970–1971 in Oxford; he was the external examiner for my doctorate, and he gave a course of lectures, for which I took notes (along with Susannah Howard), which were subsequently published in the Mathematical Institute’s Lecture Note Series. So I was in at the ground floor!

Subsequent developments

We are now faced with four kinds of objects; in order of increasing generalisation, these are association schemes (symmetric coherent configurations), commutative coherent configurations, homogeneous coherent configurations (those for which the diagonal is a single relation), and general coherent configurations. This has given rise to some problems of terminology!

In 1973, Philippe Delsarte wrote a doctoral thesis at the Université Catholique de Louvain, which was reprinted in the Philips Research Reports Supplements, and has been one of the most influential documents in the theory. Delsarte restricted himself to commutative association schemes, though in fact most of his examples are symmetric. He observed that these provide a setting for two important areas of discrete mathematics: error-correcting codes and t-designs. Codes are easier to explain here.

An association scheme is P-polynomial if there is a relation matrix R1 in the scheme which generates it, in the sense that Ri is a polynomial of degree i in R1. The graph with edge set R1 is what is known as a distance-regular graph. An important class of examples are the Hamming graphs, where the vertex set is the set of all words of length n, or n-tuples over an alphabet of size q, two vertices being joined if they differ in just one coordinate.

A d-error-correcting code is a set of words with the property that, if one is transmitted through a noisy channel and at most d symbol errors occur, then the received word is still closer to the transmitted word than to any other word in the code; so, with this assumption, the transmitted word can be found. This is just a set of words (or vertices in the Hamming graph) whose pairwise distances are all at least 2d+1. Delsarte observed that this definition could be made for any distance-regular graph, and many of the results about such codes (including the sphere-packing bound and Lloyd’s Theorem) could be proved in the more general situation. (Norman Biggs was developing similar ideas at about the same time.) Delsarte also showed that the technique of linear programming could be used to find new bounds on codes which in some cases were stronger than any previously known.

Delsarte also remarked that there was a dual concept to P-polynomial schemes (or distance-regular graphs), which he called Q-polynomial schemes; and that these schemes provided a setting for a generalisation of t-designs (in the so-called Johnson schemes) and orthogonal arrays (in the Hamming schemes).

At about this time, Eiichi Bannai and Tatsuro Ito were using similar techniques to prove the non-existence of Moore graphs with diameter greater than 2 other than odd cycles. (These graphs, which I won’t define here, would be examples of distance-regular graphs. The theorem was also proved by Damerell at about the same time.)

This led, a few years later, to a very influential book on algebraic combinatorics by Bannai and Ito, Association Schemes, I. (Volume 2 never appeared.) These authors defined association schemes to be homogeneous coherent configurations; but, like Delsarte, most of the important examples were symmetric. Their focus was on schemes which are both P- and Q-polynomial. I mentioned above that in a P-polynomial scheme, the i relation matrix is a polynomial of degree i in the first one. These polynomials satisfy a three-term recurrence relation, and so form a family of orthogonal polynomials. For the Hamming scheme, these are the Krawtchouk polynomials; for the Johnson scheme, the Eberline or dual Hahn polynomials, as Delsarte had observed. Q-polynomial schemes give a dual family of polynomials. Some of the examples led to new families of orthogonal polynomials; one of these consists of the Bannai–Ito polynomials.

Of course I cannot trace everything that has happened since. In the preceding post, I described very briefly the talks given at the International Workshop on Bannai–Ito Theory in Hangzhou last month; as well as the subjects mentioned above, there were connections with synchronization, optimality, symmetric spaces, quantum probability, and much more.

I expect that much of this will be discussed at the Pilsen meeting. But in particular, one thing that will be under discussion is the history of the subject. Before all the participants have “left”, it is important to get a record of what happened in those confused early days, when communication between researchers was by letter, or (in the case of the Soviet Union and the West) virtually nonexistent.

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Workshop on Bannai-Ito theory

The original purpose of our trip to China was to speak at an international workshop on Bannai-Ito theory at Zhejiang University in Hangzhou, a popular Chinese tourist destination.

Images of Hangzhou

Hangzhou is set on an artificial (but centuries old) lake, with islands, causeways, willows, pagodas, temples, boats, and so on. The picture above shows the Lei Feng pagoda, Xiao Ying Zhou Island, the Ling Yin temple, and a small lake in the Botanic Gardens. The first three were in places where we were taken by the generous and friendly students, Zhiwen He and Weicong Li. Xiao Ying Zhou Island is the viewpoint for the famous “Three Ponds Mirroring the Moon” which appeared on the 1 yuan note (now replaced by a coin).

Rosemary and I went a day early to give us a bit more time for sightseeing. Conference registration was on Thursday afternoon, and the talks ran from Friday morning until Sunday lunch. It was a packed schedule, with 28 talks in the two and a half days. I will try to give a brief summary. I don’t know what Bannai–Ito theory actually is, but we heard a lot about association schemes, orthogonal polynomials, and so on.

The schedule said that the first item was an opening ceremony at 8.30 followed by my talk at 9. But China apparently doesn’t go in for long opening ceremonies. At 8.30, Jack Koolen said a few words saying essentially that he was happy we had all come, and then it was announced that we would have the conference photograph. So we all traipsed outside for that, and then back in again. Then someone said, shall we wait until 9 or shall we just get started right away? The latter view prevailed, so without further ado I started my lecture. It is true that at about 9 a small group of students arrived and seemed a bit disconcerted to find the first speaker in full flow.

Conference photograph

We had been unable to use the regular conference room due to another booking, so the first day’s talks were in a room which was too small (if everyone came, some had to stand), and where the screen was too low: from the back of the room it was impossible to see the last few lines. Despite this, my talk went OK, with even a few interruptions, I’m glad to say. I realised I had left something important out of the statement of one theorem, so made a mental note to fix the slides (which involved adding a new frame).

My talk was on synchronization and separation in the Johnson association scheme, which I have discussed here recently, so I will say no more.

The second talk was by Akihiro Munemasa, who had a nice idea for an approach to permutation representations which are not multiplicity-free. He supposes that the multiplicities are at most two, and the action is imprimitive, so that, if a character has multiplicity two, then it occurs precisely once in the action on blocks. This means that in the isotypic component affording twice this character, there is a “canonical” submodule affording the component in the action on blocks, and then its perp forms a “canonical” second submodule. He was able to use this to get formulae for the minimal idempotents, and hence develop a Delsarte-type theory.

Sung-Yell Song gave us a brief account of partial geometric designs (or 1 1/2 designs according to Arnold Neumaier), with some examples which went past rather fast.

Takayuki Okuda proved a theorem which he claims has an application to isometric embeddings of compact Riemannian symmetric spaces. The theore says that if you take the “opposition involution” on a root system and produce a graph on the quotient in the obvious way, its independence number is the dimension of its fixed point space. He gave the values of this parameter for all the indecomposable root systems.

Rosemary talked about products of association schemes, especially generalized wreath products and crested products.

Then we had a lunch break, with a huge amount of food. There would have been time to do something after lunch, but I didn’t take the opportunity.

Jason Williford discussed the representation diagrams of association schemes, a concept arising naturally from the notion of Q-polynomial schemes, and gave some background.

Bill Martin talked about joint work with Brian Kodalen on the connectivity of the graphs in an association scheme. The obvious conjectures are that both vertex and edge connectivity should be equal to valency, and the only disconnecting sets of the appropriate sizes should be those which isolate a vertex. They can prove that the condition that the graph obtained by deleting a closed neighbourhood is connected is equivalent to there being no two vertices with the same neighbourhoods.

Yan Zhu talked about the structure of shells in the Hamming schemes (the sets of points at fixed distance from a given point), and the question of what t-designs in these schemes look like. A shell in the Johnson scheme is just the direct product of smaller Johnson schemes, but shells in the Hamming scheme are more interesting. (In Rosemary’s terms, there is “something” nested in a Johnson scheme, but exactly what it is is not so clear.) This was joint work with Eiichi Bannai.

Da Zhao presented a theorem, also joint with Bannai, classifying the association schemes which have spherical embeddings in 3-dimensional Euclidean space: there are just seven of these, the five platonic solids and the line graphs of the cube and dodecahedron.

The final session was the highlight, with talks by Tatsuro Ito and Eiichi Bannai. Ito spoke about “Bannai’s Jugendtraum“, which (he said) was to give an alternative approach to the classification of finite simple groups by means of association schemes. The conjugacy class scheme of a finite group is a commutative scheme which is primitive if and only if the group is simple. If we could characterise up to parameters the schemes that arise, then we would just have to characterise all the finite simple groups by their character tables. This dream was not realised and probably is unlikely to be. But this led to P- and Q-polynomial association schemes, Leonard’s theorem and the connection with orthogonal polynomials, and Terwilliger’s theory; there a classification is in sight, as Ito and Terwilliger are on the case. (Also, of course, it led to the celebrated book by Bannai and Ito.)

Then Bannai spoke. He described how, as a student of Iwahori, he had become very familiar with the commutative square

commutative diagram

He worked on permutation representations of the groups on the left of the diagram. This led him and Ito to the theorem on Moore graphs (simultaneous with Damerell), association schmes, and the book with Ito, which of course stimulated much more work. He concluded with some problems he still dreams of solving: a bound on the degree of transitivity not using CFSG; re-doing CFSG using algebraic combinatorics (he admitted that these two are out of reach); a theory of commutative association schemes following that of compact symmetric spaces; and generalising tight t-designs to other contexts. In particular, he would like multivariate versions of P- and Q-polynomial association schemes and of Askey–Wilson polynomials.

Then it was time for the conference banquet, where we ate and ate, and finally
staggered home to bed.

The next day, we had to be early again, since we needed to follow the crocodile to the new conference room (though I had a good idea where it was, and could have found it myself). This was in the Sir Run-Run Shaw building; the door we entered said it was the Business Administration department, though the door on the ground floor called it the Mathematics Department (in common with the building we were in yesterday).

We were in a much bigger room, but still not ideal; the room was not raked, so it was still a bit difficult to see the bottom of the screen; and there was storage space under the desks, which meant that my legs didn’t go under unless I twisted them into an uncomfortable contortion.

Kaishun Wang kicked off the proceedings with a talk on weakly distance-transitive digraphs. This is where the relation of two points specifies their distance in both directions, and it is required that these relations form an association scheme. He had cclassifications in some cases including valency 2 or 3, thin case (all nontrivial intersection numbers at most 1), and partial results on diameter 2.

Hajime Tanaka came from quantum probability theory. Essentially, he looks at the spectral distribution of various families, suitably normalised: e.g. for Hamming, he gets Poisson if q/n tends to a finite limit, or normal if it tends to zero; the corresponding orthogonal polynomials are Charlier and Hermite polynomials. Similarly, Johnson schemes give geometric and exponential distributions and Meixner and Laguerre polynomials.

He went on to look for multivariate analogues. In particular, the limit of the spectral distribution of symmetric products of strongly regular graphs, suitably normalised, gives either bivariate Gaussian, or priduct of Gaussian and Poisson, or bivariate Poisson, depending on various limits; he can describe the orthogonal polynomials also.

Sho Suda found the invariant factors of a skew-symmetric ±1 matrix of order 4n+2, as given by the Ehlich–Wojtas theory. As a corollary, Kharaghani’s example of a matrix of order 66 meeting the determinant bound cannot be equivalent to a skew-symmetric matrix.

Zhi Qiao gave a characterisation of dual polar spaces of sufficiently large
diameter: he requires large cliques (meeting the Delsarte bound) and local
conditions on the association scheme, not the full strength of this.

Wei-Hsuan Yu has been improving bounds for numbers of equiangular lines in Euclidean space, and has resolved a number of unknowns in Seidel’s table. But not everything is known yet. He showed that the 90 lines in 20 dimensions cannot have another line added; this is of course not equivalent to showing that 90 is the maximum. The week after the workshop, he came to SJTU to give a seminar, and went into a bit more detail about the semidefinite programming used.

Ziqing Xiang talked about explicit constructions of spherical designs. The existing constructions for given strength and dimension depend on continuity arguments and so are not explicit; he has managed, in a remarkable construction, to give explicit rational constructions, though the price he pays is that the designs are rather large!

Then the lunch break. After lunch, taking advice from Yaokun Wu and the students, Rosemary and I went to the botanic gardens. After paying the small entrance fee, we went up a path over a hill and down the other side, to an area with small lakes with beautifully coloured leaves on the trees and picturesque buildings around them, together with the other side, car parks and huge restaurants.

Paul Terwilliger started the afternoon with a talk on Leonard triples (a new development from Leonard pairs; I am not sure of its application to association schemes). He has constructed three matrices which he calls “pseudo-intertwiners” for the triple, and took us through the rather complicated algebra necessary to verify their properties.

Suogang Guo used the Terwilliger algebra for the folded cube (which is in this case just the coherent algebra for the point stabiliser in its automorphism group – I don’t know how general this is!) to get improved bounds for codes in this graph. However, he didn’t actually show us an explicit example where his method beats known bounds.

Hiroshi Nozaki was interested in bounding the number of vertices in a connected bipartite graph with given valency and given second eigenvalue. On a similar theme, Jongyook Park talked in some generality about the second eigenvalue of a distance-transitive graph and what it tells you.

Sergey Goryainov talked about perfect 2-colourings (that is, equitable partitions with two parts) of the bilinear forms graph of dimension 2×d over the field of two elements. For such a partition, the 2×2 collapsed adjacency matrix has two eigenvalues, one of them the valency; in the case where the other is the second eigenvalue, he was able to give a complete classification. More on this later.

The day ended with Alexei Zhedanov talking about orthogonal polynomials on the unit circle and their connection with double affine Hecke algebras, things which had been mentioned in other talks. He says that these things have applications in solving certain multivariate eigenvalue problems, such as for (α1R12R2I)f = 0, which physicists and others are interested in. The polynomials are the Szegő orthogonal polynomials.

The next morning we started a bit later, since we knew the way, but found when we arrived right on time that Eric Moorhouse had just started. Fortunately we didn’t miss much. Eric gave a beautiful talk on double covers, his tool of first resort for many mathematical problems. He showed us by way of introduction that the cube and dodecahedron are (antipodal) double covers of K4 and the Petersen graph, and that there is a repeated double cover from K4,4 to the 4-cube to a 4-regular graph of girth 6 which is most of PG(2,4). A similar double extension leads from K6 to the icosahedron to a graph which is most of PG(2,5). Also, the folded 6-cube is a double cover of both the 6-valent strongly regular graphs on 16 points.

There are 193 known projective planes of order 25, falling into 99 dual pairs. Eric analysed these, taking the quotient of a plane by an involution (discarding fixed points) and then a double cover of this. He found several examples, none of them new. He is trying 49, but there are millions of planes there! He also used an idea due to Conway to get an efficient isomorphism invariant for things involving permutations (such as loops, nonincidence graphs of projective planes, etc.)

He has used double covers of complete graphs (that is, two-graphs – he used this term) to get an isomorphism invariant for ovoids, and show the nonexistence of ovoids in large odd dimensions over odd characteristic.

The piece de resistance wsa the set of Lagrangians (maximal totally isotropic subspaces) in a symplectic space in odd characteristic. There is a 2-valued invariant of triples of Lagrangians called the Maslov index, which is a two-graph and so gives a double cover. He explained how this is constructed in the case of pairwise disjoint Lagrangians X,Y,Z. Any vector in Z has the form x+f(x), where f is an isomorphism from X to Y; using the fact that Z is t.i. and the form is alternating, you find that B(x,f(y)) = B(y,f(x)), so you get a symmetric bilinear form on X, and take the type of the corresponding quadratic form in the Witt ring. Interesting stuff!

Edwin van Dam classified graphs with an eigenvalue of multiplicity 3 which are partially metric (that is, they generate an association scheme, and the distance 2 relation is a single relation in the scheme. APart from complete multipartite graphs, there are just nine such, with interesting relationships, e.g. the Foster graph is a double cover of the Nauru graph.

Hirotake Kurihara comes from differential geometry. He showed us how a irreducible Hermitian symmetric space has a “scaffold” which is a distance-transitive graph. Such a graph is built on the fixed point set of the isometry sx fixing the point x in the definition of symmetric space. It is uniquely determined by the space, and conversely. Grassmann spaces give rise to Johnson graphs. The two exceptional examples give the Schläfli and Gosset graphs.

Wei Wang talked about the graph isomorphism problem, specifically, is it true that almost all graphs are determined by their spectrum (as Willem Haemers conjectured)? He has partial results when you give yourself the spectra of the graph and its complement. He mentioned two things I didn’t know: W. X. Du has improved Babai’s quasipolynomial bound for graph isomorphism to an c log n bound; and Tao and others have shown that almost all graphs have a simple eigenvalue.

Finally Jack Koolen gave a lovely talk about representing graphs with least eigenvalue −3 or greater by integral lattices with minimum norm −3. Following Conway and Sloane, a lattice L is called s-integrable if (√s)L is isomorphic to a sublattice of the standard lattice Zn. Thus E6, E7 and E8 are 2-integrable but not 1-integrable. His main theorem asserts that there is a number K such that a graph with minimum valency at least K and least eigenvalue at least −3 is 2-integrable. (This extends Hoffman’s theorem, which has 2 and 1 in place of 3 and 2.) He pointed out that, although the upper bound in the theorem is huge and can surely be improved, the true bound cannot be smaller than 163, since the strongly regular graph of valency 162 on 275 vertices is not 2-integrable. He conjectures that the main theorem holds also for signed graphs, and that all such graphs and signed graphs are s-integrable for some bounded s (perhaps the correct bound is 4).

Then it was the end of the conference. The organisers were thanked; in fact Tao Feng and his team were out of the room when the thanks began, but were quickly fetched. Rosemary and I went for a coffee, then went for a walk by the lake, on the Su causeway, and on Gushan island, before heading back to the hotel where taxis would be fetched to take us to the station for the train back to Shanghai.

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Spotted, 2

Erdos Plaza

I hadn’t realised he was so famous in Shanghai …

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Spotted

On the Shanghai metro yesterday, we saw a man with a T-shirt bearing the caption

ANTI SOCIAL SOCIAL CLUB

Do I smell a trace of Cantor’s diagonal argument here? Perhaps even Russell’s paradox? Suppose that every social club is named after a person, and a person X is antisocial if X is not a member of the X club. Then let the club of antisocial people be the Y club, and ask: Is Y social or antisocial?

Perhaps you would like to formalise this …

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Xu Guangqi

Xu Guangxi

Our hotel in Shanghai is very close to the tomb of Xu Guangqi (1562-1633), Chinese Renaissance man: mathematician, first Chinese translator of Euclid, astronomer, calendar reformer, military strategist, agronomist, and more.

Just across the road from our hotel is the site of his observatory, and outside is a fine statue of him (pictured above). Along at the next corner is a tiny park which contains more statues, his tomb (with a sacred road to it from the entrance gate), more statues (including other facets of his life: one with a telescope, one with a European cannon, one planting sweet potatoes), and a small museum about him (which contains his Euclid translation, apparently, though it was closed when we visited the park). The trees are full of singing birds, quite extraordinary in this busy built-up part of the huge city, and the pond contains turtles and huge goldfish. In the park, people play cards (very many of these), do tai chi exercises, or stroll around photographing the memorials on their phones.

Xu was converted, both to a belief in the superiority of Western science, and to Christianity, by the Jesuit scholar Matteo Ricci, and gave the land on which the Catholic cathedral is built, also across the street from our hotel. Around here there were many monasteries. Indeed, we dined in a very good restaurant called “Ye Olde Station Restaurant” which, despite its name, was previously a nunnery, and also contains many old artefacts such as sewing machines and cameras.

You can read the St Andrews MacTutor account of Xu’s life and work here.

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In Shanghai

Shanghai

On Friday afternoon, at the LMS meeting, I received from the outgoing President Simon Tavaré the certificate for the Senior Whitehead Prize. Simon gave a delightful Presidential Address, but now is not the moment to describe it. Then a very pleasant occasion at dinner after the meeting.

On Saturday we packed and went to the airport to catch a China Eastern Airlines flight to Shanghai, as guests of Yaokun Wu at Shanghai Jiao Tong University.

So this morning we woke up in a hotel in Shanghai.

Reports may be intermittent for a while. Last night I discovered that some things work and others don’t; was it the conspiracy theory (blocked by the great Chinese firewall) or the cockup theory (the rather pathetic hotel wi-fi)? As I would expect, it was the latter. So I can connect to WordPress, but it is rather slow and painful.

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