Evoa

Lapwings

A very different kind of excursion last Sunday, to Evoa, a nature reserve near the Tejo estuary.

Quite a drive from Lisboa: along the highway to Vila Franca, and the iron bridge which was the last bridge across the Tejo before the two new bridges in the city were built; over the bridge; then 12.5km along the kind of road you find in the Australian outback (gravel, potholed and corrugated) to the reserve. Along the road we saw several creatures which we took at first to be huge scorpions; we learned later that they were Louisiana freshwater lobsters, introduced by the Spanish (who apparently liked to eat them) – now, it seems, storks like to eat them.

Lobster

The land here is dead flat, in contrast to the hills on the other side of the river. Farmers grow rice and raise fine black horses and bulls for the bullring. Among the horses we saw many white egrets, with whom they have a symbiotic relationship.

The weather had changed after the cool damp days of my visit so far, and it was quite hot as we went for about a 5km walk around the several lakes of the nature reserve, along tracks with tall reeds towering overhead. From the reeds came a lovely warbling sound, and indeed a reed warbler was sitting on a sign to welcome us to the first hide. (Their Portuguese name means “nightingale”; I have never heard a nightingale, so I don’t know how the song compares.)

We stopped in several hides and saw a variety of waterbirds. Among them were gulls, terns, grey herons, white egrets, mallard and teal ducks, coots, avocets, stilts, and for me the highlight: a spoonbill, holding its bill just under the water and waving its head from side to side to catch small aquatic creatures.

After some delay when the car refused to start, we left along the same track, seeing many more herons and egrets and a couple of hawks.

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Tomar

Tomar

The Portuguese city of Tomar was founded, on the site of a Roman town, by the Knights Templar in the twelfth century. (Many businesses in the town still use the Templar name or their logo, which is also found in the calçada pavements throught the town.) The town lies on the Nabão River (the name means “turnip”, I don’t know why).

It was saved from the wreck of the Templars (they had grown too rich and powerful and were brought down by an alliance of the Pope and the King of France who wanted to get his hands on their treasure) by King Dinis of Portugal, who had the town and its assets transferred to a newly created Order of Christ.

The famous Portuguese prince Henry the Navigator became head of the Order. It may be that some of its wealth funded the journeys of exploration and discovery made by the Portuguese during his reign.

The focus of the town, standing on top of the highest hill, is the castle and convent of the Order of Christ, with a 12th century round chapel said to be modelled on the Temple of Jerusalem. Later, additional building works were carried on in the Manueline period, showing the characteristic armillary sphere and naval ropes in the stonework. This destroyed the austere simplicity of the chapel, but contains a window which has become the symbol of Tomar. Water was supplied by a 6-kilometre aqueduct. The hilltop is now a Unesco World Heritage Site.

Manueline window

We went there last Saturday, primarily to visit the Convent. After wandering down the long dormitory corridors, and through the gardens with sweet-smelling lavender, we went down to the town in search of refreshment. Despite some difficulty finding a parking place, we ended up in a small café.

Every Portuguese town with a convent has at least one speciality sweet cake, and Tomar is no exception: I had a cake which looked like a large slice of mango but tasted like the food of angels. (I recently found out the reason for this. The convents consumed large numbers of eggs – they used the whites to stiffen the nuns’ wimples – and they had to find a use for the yolks.)

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Comics, literacy and self-help

Neill recently spent a week pouring out his passion and commitment to children’s comics on his blog. I thought it made remarkable reading. It is indexed on a side panel, or you can get the series from Readlists (a site I didn’t know about) or as an ePublication from Amazon.

In short, his argument is that a serious decline in children’s literacy has coincided with a catastrophic decline in the availablity of cheap children’s comics; everyone says we should do something about literacy, and comics have a big part to play in any such campaign. But rather than just bewailing the fact that publishers no longer produce, and corner stores and newsagents no longer stock, comics for kids, there is something else we can do: encourage children to produce their own comics, and help them with alternative distribution channels, which might be on the Internet, or comics clubs in schools or local libraries, or whatever.

His own observations of doing workshops for children (and living with one) make it clear that children love reading comics, even (or especially) comics by other children, and with a bit of encouragement they can become engaged in both the creative and the entrepreneurial side of producing them. Moreover, there are books about how to do it that can be put into their hands, and talented people around who run workshops.

Why am I mentioning this here? I think there are some surprising similarities, as well as some differences, with mathematical publishing. If you look at what I wrote, and substitute “mathematicians” and “theorems” for “children” and “comics”, a lot of it makes sense. We love reading theorems, especially if the presentation as well as the content are creatively done; our job is proving new theorems and crafting presentations of them; and some mathematicians at least can become engaged in the distribution side. In particular, I’d like to pay tribute to Herb Wilf and Neil Calkin, who founded the Electronic Journal of Combinatorics.

What are the differences? With us it is not that publishers will not produce the stuff and put it into our hands; but what they ask (for subscription or page charges) is rather higher than “prices realistically within the realm of pocket money while still maybe even leaving enough change for a bag of Skips”, as Neill puts it. So we can’t get them ourselves, and depend on our employers to buy them for us, or to pay the steep entry fee for us to publish in them (which means putting power over what we read and where we publish into the hands of the bureaucrats).

Of course, what keeps us locked in is the insistence of bureaucrats that we publish only in “approved” journals (approved by whom? by them, of course, not by us) or our publication won’t count in evaluations of our research (evaluations by whom?).

Here I would echo Neill and say, we can wring our hands and bewail the situation, or take what steps we can to remedy it. Expertise in running freely available journals is available; we should use it. If those of us who are no longer subject to these stupid bureaucratic rules support this enterprise, eventually “they” will not be able to ignore these outlets.

So I wish the comics creators every success, and at the same time I wish every success to the committed and creative people who are trying to provide us with outlets for our best work. Support them! Help them if you can, and send them good papers.

As a final note, I am delighted when Neill’s world and mine link up in this way.

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9,21,27,45,81,153,…

This is the sequence of degrees of primitive groups which don’t synchronize a map of rank 3, equivalently graphs with clique number and chromatic number 3 having primitive automorphism groups. You could argue that the sequence should start with 3, but this is a trivial case.

The numbers must all be multiples of 3, since the colouring has all colour classes of equal size. But all numbers above are odd, and all except 21 are multiples of 9; I don’t know why this is.

How does the sequence go on? It contains 243, 441, 729, … but I don’t know if there are other terms before these.

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Butterflies

I am in Lisbon working with João Araújo and Wolfram Bentz on synchronization.

We say that a permutation group G on the set {1,…n} synchronizes a non-permutation f from this set to itself if the semigroup generated by G and f contains a constant map. Also, the kernel classes of a map f are the inverse images of the points in the image of f; and f is uniform if all the kernel classes have the same size, and non-uniform otherwise.

A group that synchronizes every non-permutation must be primitive. It is not true that a primitive group synchronizes every non-permutation; there are some very interesting exceptions, and deciding the question is very difficult. The current “big conjecture” on synchronization for primitive permutation groups is due to João, and says:

Conjecture: A primitive group synchronizes every non-uniform map.

I have spoken about this conjecture in several places. We had proved it for maps of rank at most 4 (where the rank is the cardinality of the image) or at least n−2, in a paper published this year in the Journal of Combinatorial Theory Series B (doi: 10.1016/j.jctb.2014.01.006). We are currently hard at work improving the upper bound, and hope to get it down to n−4 or even n−5.

This morning, we discovered that the conjecture is false.

Here is a brief description of the counterexample.

The graph in question is the line graph of the Tutte–Coxeter graph, also known as Tutte’s 8-cage. The Tutte–Coxeter graph is trivalent; its line graph has valency 4, and any edge lies in a unique triangle (the triangles corresponding to the vertices in the original graph), so the closed neighbourhood of a vertex is a butterfly, consisting of two triangles with a common vertex.

Butterfly

As in dynamics, it is the butterfly that causes chaos with a flap of its wings …

Our graph has automorphism group G = PΓL(2,9), and has chromatic number 3; this means that it has a homomorphism onto one of its triangles, this being a (uniform) map of rank 3 not synchronized by the primitive group G.

We used GAP and its share package GRAPE to determine all the independent sets of size 15 in the graph (up to the action of the group G, there are just two of these), and to examine the induced subgraph on the complement of each such set. In both cases, this induced subgraph turns out to be a disjoint union of cycles of even length; so each independent set of size 15 is a colour class in a 3-colouring. For one of these independent sets, it occurs that the induced subgraph on the complement has two components, of sizes 10 and 20. In this case, the original independent set A and the bipartite blocks B and C of the component of size 10 and D and E of the other component are all independent sets, with edges between A and the others, and between B and C and between D and E, and no further edges.

Thus the edges between these five sets can be mapped homomorphically to the butterfly, with A mapping to the central vertex. This gives us a non-uniform map of rank 5 (with kernel classes of sizes 15, 5, 5, 10, 10) which is an endomorphism of the graph, and hence not synchronized by the group G.

Indeed, this butterfly resembles Lorenz’s attractor in one respect. If you wander round the graph and follow your image under the homomorphism, it will move around one wing of the butterfly and then (apparently randomly) switch to the other wing, and so on. Actually, since the wings are of unequal sizes, you will spend most of your time on the larger wing.

Is this an isolated example, or the first butterfly of the summer? (It is not completely isolated; the line graph of the Biggs–Smith graph gives another example with degree 153, where the kernel type is (6,6,45,45,51).)

Butterfly

As with any good counterexample, it opens various new questions. Is it the smallest counterexample? What can we say about the gap between the ranks of the smallest map and the smallest non-uniform map not synchronized by a primitive group? (It can’t be 1; how large can it be?) Does a primitive group of degree n synchronize every map of rank greater than n/2? Can one determine all the primitive groups which fail to synchronize some map of rank 3? And so on …

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10th birthday of MSG

Another event last week was the 10th anniversary talk in the Mathematics Study Group at South Bank University.

This group was set up by Carrie Rutherford, whose photo is below (as well as a mathematician at South Bank, she is a volunteer on the Markfield beam engine near Tottenham).

Carrie Rutherford

Carrie learned how to run a study group as a member of the Combinatorics Study Group at Queen Mary. She set up her own at South Bank, and it has evolved its individual style, attracting a range of people from as far afield as Norwich and Portsmouth.

The first ever talk in the MSG was on Hadamard matrices, and when Carrie asked me if I could talk about Hadamard matrices I was happy to comply.

Hadamard asked the question:

How large can the determinant of an n×n real matrix be, if its entries all have absolute values at most 1?

There is a simple geometric argument. The determinant is the volume of the Euclidean paralleleliped spanned by the rows of the matrix. The Euclidean length of each row is at most n1/2, and the volume spanned by vectors of fixed length is maximised when the vectors are pairwise orthogonal; so the maximum determinant is nn/2. Equality is achieved if and only if all the entries in the matrix H are +1 or −1 and HHT = nI. Such a matrix is called a Hadamard matrix. (This is a nice example of how the solution to a continuous problem may plunge you into the discrete world.)

It is easy to show that the order of a Hadamard matrix must be 1, 2 or a multiple of 4. It is conjectured that every multiple of 4 is the order of a Hadamard matrix. This is one of the big open problems in discrete mathematics. I think that the first unsettled case is order 668. (The previous smallest, 428, was solved at the IPM in Tehran shortly after my visit there in 2005.)

There has been some interest in symmetric and skew Hadamard matrices. (A Hadamard matrix can’t really be skew, since a real skew matrix has zero diagonal; we abuse language by saying that a skew-Hadamard matrix is one with +1 on the diagonal and HI skew-symmetric.)

One could ask Hadamard’s question also for matrices with zero diagonal and off-diagonal entries with absolute value at most 1. The same argument shows that the determinant of such a matrix C is at most (n−1)n/2, with equality if and only if the off-diagonal entries of C are +1 or −1 and CCT = (n−1)I. A matrix meeting the bound is called a conference matrix. (The name comes from their occurrence in conference telephony in the 1950s.)

One of the most remarkable facts about conference matrices is that, essentially, such a matrix must be either symmetric or (genuinely) skew-symmetric:

Theorem: A conference matrix of order greater than 1 has even order, and is equivalent (under row and column sign changes) to a symmetric matrix if n is congruent to 2 (mod 4), or to a skew-symmetric matrix if n is congruent to 0 (mod 4).

It is easy to see that C is a skew conference matrix if and only if C+I is a skew-Hadamard matrix, so the existence conjecture in this case is that both types exist for all multiples of 4. In the symmetric case, however, van Lint and Seidel showed that a symmetric conference matrix of order n exists if and only if n−1 is the sum of two squares.

Symmetric conference matrices are obtained by bordering with 1s the Seidel adjacency matrices of Paley graphs. Now the South Bank University logo is a pentagon, and the Mathematics Study Group logo the 3×3 grid; these are the first two Paley graphs, so give rise to conference matrices of orders 6 and 10, the latter very fitting for the anniversary of the MSG!

A similar construction shows that skew-Hadamard matrices are obtained by suitably bordering the signed adjacency matrices of doubly regular tournaments. By coincidence, these arose in another piece of work I put on the arXiv recently, joint with statisticians from the UK, Germany and Poland on circular repeated-measurements designs.

Also from statistics is an intriguing conjecture by Denis Lin on the relation between maximum determinant of skew matrices with orders congruent to 2 (mod 4), similar to the relation H = C+I that we saw in the case where the order is congruent to 0 mod 4. However, for this, I will refer you to the slides, which are on the on the MSG webpage, or in the usual place.

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Queen Mary MathSoc

In my long association with Queen Mary, one constant has been the variability of the Queen Mary Mathematics Society. As with many student societies, it takes an enthusiastic student or two to breathe life into it, and when these people leave it returns to somnolence.

Maybe it is different this time …

Last Tuesday I addressed the newly revitalised Queen Mary Mathsoc, at the invitation of Giulia Campolo. Perhaps she brings Italian flair to the job; in any case, she had produced a T-shirt with a logo devised by an Italian designer, and the result is impressive (if you can ignore the model).

Queen Mary MathSoc T-shirt

I particularly like the choice of logo, which is described on the back as
follows:

The crumpled paper

A humble reminder of the frustration of all mathematicians, their effort and dedication in pursuit of a new solution, a breakthrough formula, or a lifetime chimera.

A dimension for human speculation and a place for abstraction, where both victory and failure are a possibility.

The second and third year students took my Mathematical Structures course. I don’t claim that I taught them the appreciation of mathematics which this logo and its description show; but I hope I contributed in some degree.

Anyway, I really enjoyed the evening. I had an audience of close to 100; this included first-year students as well as students I had taught, as well as others from computer science, physics, even genetics. I talked from 6 till 7, and afterwards we sat around and chatted, and it wasn’t until 9 that I noticed it was getting late and I should go.

I talked about Paradox. I wanted to get across my view, which is that rather than the famous paradoxes (infinitesimals, Russell’s paradox, Gödel’s Theorem, and the rest) being destructive of mathematics, they greatly enrich it by showing new aspects of our playpen, much as non-Euclidean geometry did. The three examples above gave us calculus, axiomatic set theory, and non-standard arithmetic and analysis.

Added 13 October: Here is a picture taken after the lecture by Martyna Sikora:

Lecture to Mathsoc

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