# Category Archives: exposition

a post aimed to teach something

## London Combinatorics Colloquia 2023

In London last week for this (usually) annual event, now happily back live. In contrast to usual practice, I will discuss the second day first. Norman Biggs was the founder of the combinatorics group at the London School of Economics, … Continue reading

## Bases

A base for a permutation group is a sequence (a1,…,ar) of points in the permutation domain whose pointwise stabiliser is the identity. Bases were introduced in computational group theory by Charles Sims, since two elements of a permutation group are … Continue reading

## Orthogonal block structures

If you google “commuting equivalence relations” you will find signs of a large and rich literature. I want to talk about one aspect, on the border of algebraic combinatorics and experimental design: orthogonal block structures. Look up orthogonal block structures … Continue reading

Posted in doing mathematics, exposition | | 1 Comment

## Between transitive and primitive

Many familiar properties of permutation groups form a hierarchy. A few years ago, with João Araújo and Ben Steinberg, I wrote a survey on properties related to synchronization of finite automota, lying between primitivity and 2-transitivity. Here is a new … Continue reading

## Mathematical Structures

My last major job at Queen Mary University of London more than ten years ago was designing and presenting a new first-semester first-year module to be taken by all students on mathematics programmes or joint programmes involving mathematics. I discussed … Continue reading

Posted in exposition, teaching | | 4 Comments

## Two things that should be related

Here are two things that look as if there should be a relation between them. A graph duality In my paper on graphs defined on groups, I invented an ad hoc duality relation between pairs of graphs. The context is … Continue reading

## A theorem on polytopes

You know what polygons and polyhedra are. How do we extend their study to higher dimensions? There are two parts to this question. The first involves incidence geometry: vertices, edges, faces, etc. Here the generalisation is fairly straightforward. A polygon … Continue reading

Posted in doing mathematics, exposition | | 3 Comments