Suppose we have a finite graph. We can view this as a resistor network where each edge has unit resistance. We can then calculate the resistance between any two vertices and ask questions like `which graph with $n$ vertices and $m$ edges minimises the average resistance between pairs of vertices?' There is a `obvious' solution; we show that this answer is not correct.

This problem was motivated by some questions about the design of statistical experiments (and has some surprising applications in chemistry) but this talk will not assume any statistical knowledge.

This is joint work with Robert Johnson.

# Past Combinatorial Theory Seminar

Erdős asked the following question: given a positive integer $n$, what is the largest integer $k$ such that any set of $n$ points in a plane, with no $4$ on a line, contains $k$ points no $3$ of which are collinear? Füredi proved that $k = o(n)$. Cardinal, Toth and Wood extended this result to $\mathbb{R}^3$, finding sets of $n$ points with no $5$ on a plane whose subsets with no $4$ points on a plane have size $o(n)$, and asked the question for the higher dimensions. For given $n$, let $k$ be largest integer such that any set of $n$ points in $\mathbb{R}^d$ with no more than $d + 1$ cohyperplanar points, has $k$ points with no $d + 1$ on a hyperplane. Is $k = o(n)$? We prove that $k = o(n)$ for any fixed $d \geq 3$.

The Number Field Sieve is the current practical and theoretical state of the art algorithm for factoring. Unfortunately, there has been no rigorous analysis of this type of algorithm. We randomise key aspects of the number theory, and prove that in this variant congruences of squares are formed in expected time $L(1/3, 2.88)$. These results are tightly coupled to recent progress on the distribution of smooth numbers, and we provide additional tools to turn progress on these problems into improved bounds.

Given two probability distributions $P_R$ and $P_B$ on the positive reals with finite means, colour the real line alternately with red and blue intervals so that the lengths of the red intervals have distribution $P_R$, the lengths of the blue intervals have distribution $P_B$, and distinct intervals have independent lengths. Now iteratively update this colouring of the line by coalescing intervals: change the colour of any interval that is surrounded by longer intervals so that these three consecutive intervals subsequently form a single monochromatic interval. Say that a colour (either red or blue) `wins' if every point of the line is eventually of that colour. I will attempt to answer the following question: under what natural conditions on the distributions is one of the colours almost surely guaranteed to win?

**Gyárfás** conjectured in 1985 that if $G$ is a graph with no induced cycle of odd length at least 5, then the chromatic number of $G$ is bounded by a function of its clique number. We prove this conjecture. Joint work with Paul Seymour.

More than twenty years ago Erdős conjectured that a triangle-free graph $G$ of chromatic number $k$ contains cycles of at least $k^{2−o(1)}$ different lengths. In this talk we prove this conjecture in a stronger form, showing that every such $G$ contains cycles of $ck^2\log k$ consecutive lengths, which is tight. Our approach can be also used to give new bounds on the number of different cycle lengths for other monotone classes of $k$-chromatic graphs, i.e., clique-free graphs and graphs without odd cycles.

Joint work with A. Kostochka and J. Verstraete.