Theta blocks related to root systems

Gritsenko, Skoruppa and Zagier associated to a root system $R$ a theta block $\vartheta_R$, which is a Jacobi form of lattice index. We classify the theta blocks $\vartheta_R$ of $q$-order $1$ and show that their Gritsenko lift is a strongly-reflective Borcherds product of singular weight, which is related to Conway's group $\operatorname{Co}_0$. As a corollary we obtain a proof of the theta block conjecture by Gritsenko, Poor and Yuen for the pure theta blocks obtained as specializations of the functions $\vartheta_R$.Comment: Final version, published in Math. Ann.; updated acknowledgemen

Some notes on impartial games and NIM dimension

Tese de doutoramento, Matemática (Análise Numérica e Matemática Computacional), Universidade de Lisboa, Faculdade de Ciências, 2010.Disponível no documento

Closure algorithms and the star-height problem of regular languages

Imperial Users onl

Computing backwards with Game of Life, part 1: wires and circuits

Conway's Game of Life is a two-dimensional cellular automaton. As a dynamical system, it is well-known to be computationally universal, i.e.\ capable of simulating an arbitrary Turing machine. We show that in a sense taking a single backwards step of Game of Life is a computationally universal process, by constructing patterns whose preimage computation encodes an arbitrary circuit-satisfaction problem, or (equivalently) any tiling problem. As a corollary, we obtain for example that the set of orphans is coNP-complete, exhibit a $6210 \times 37800$-periodic configuration that admits a preimage but no periodic one, and prove that the existence of a preimage for a periodic point is undecidable. Our constructions were obtained by a combination of computer searches and manual design.Comment: 28 pages, 10 figures in main text. 11 pages, 20 figures in appendix. Accompanied by two GitHub repositories containing programs and auxiliary dat

Conway’s Circle Theorem: a short proof, enabling generalization to polygons

John Conway’s Circle Theorem is a gem of plane geometry: the six points formed by continuing the sides of a triangle beyond every vertex by the length of its opposite side, are concyclic. The theorem has attracted several proofs, even adorned Mathcamp T-shirts. We present a short proof that views the extended sides as equal tangents of the incircle, a perspective that enables generalization to polygons.Accepted manuscrip

On Kleene algebras of ternary co-relations

In this paper we investigate identities satisfied by a class of algebras made of ternary co-relations - contravariant ("arrow-reversed") analogues of binary relations. These algebras are equipped with the operations of union, co-relational composition, iteration, converse and the empty co-relation and the so-called diagonal co-relation as constants. Our first result is that the converse-free part of the corresponding equational theory consists precisely of Kleenean equations for relations, or, equivalently, for (regular) languages. However, the rest of the equations, involving the symbol of the converse, are relatively axiomatized by involution axioms only, so that the co-relational converse behaves more like the reversal of languages, rather than the relational converse. Actually, the language reversal is explicitely used to prove this result. Therefore, we conclude that co-relations can offer a better framework than relations for the mathematical modeling of formal languages, as well as many other notions from computer science