Succinct Population Protocols for Presburger Arithmetic

International audienceIn [5], Angluin et al. proved that population protocols compute exactly the predicates definable in Presburger arithmetic (PA), the first-order theory of addition. As part of this result, they presented a procedure that translates any formula $ϕ$ of quantifier-free PA with remainder predicates (which has the same expressive power as full PA) into a population protocol with $2 O(poly(|ϕ|))$ states that computes $ϕ$. More precisely, the number of states of the protocol is exponential in both the bit length of the largest coefficient in the formula, and the number of nodes of its syntax tree. In this paper, we prove that every formula $ϕ$ of quantifier-free PA with remainder predicates is computable by a leaderless population protocol with $O(poly(|ϕ|))$ states. Our proof is based on several new constructions, which may be of independent interest. Given a formula $ϕ$ of quantifier-free PA with remainder predicates, a first construction produces a succinct protocol (with $O(|ϕ| 3)$ leaders) that computes ϕ; this completes the work initiated in [8], where we constructed such protocols for a fragment of PA. For large enough inputs, we can get rid of these leaders. If the input is not large enough, then it is small, and we design another construction producing a succinct protocol with one leader that computes $ϕ$. Our last construction gets rid of this leader for small inputs

Roth's Theorem in the Piatetski-Shapiro primes

Let $\mathbf{P}$ denote the set of prime numbers and, for an appropriate function $h$, define a set $\mathbf{P}_{h}=\{p\in\mathbf{P}: \exists_{n\in\mathbb{N}}\ p=\lfloor h(n)\rfloor\}$. The aim of this paper is to show that every subset of $\mathbf{P}_{h}$ having positive relative upper density contains a nontrivial three-term arithmetic progression. In particular the set of Piatetski--Shapiro primes of fixed type $71/72<\gamma<1$, i.e. $\{p\in\mathbf{P}: \exists_{n\in\mathbb{N}}\ p=\lfloor n^{1/\gamma}\rfloor\}$ has this feature. We show this by proving the counterpart of Bourgain--Green's restriction theorem for the set $\mathbf{P}_{h}$.Comment: Accepted for publication in Revista Matematica Iberoamerican

Non-finite axiomatizability of first-order Peano Arithmetic

Treballs Finals de Grau de Matemàtiques, Facultat de Matemàtiques, Universitat de Barcelona, Any: 2020, Director: Enrique Casanovas Ruiz-Fornells[en] The system of Peano Arithmetic is a system more than enough for proving almost all statements of the natural numbers. We will work with a version of this system adapted to first-order logic, denoted as PA. The aim of this work will be showing that there is no equivalent finitely axiomatizable system. In order to do this, we will introduce some concepts about the complexity of formulas and codification of sequences to prove Ryll-Nardzewski’s theorem, which states that there is no consistent extension of PA finitely axiomatized

On dimensions of block algebras

Following a question by B. K¨ulshammer, we show that an inequality, due to Brauer, involving the dimension of a block algebra, has an analogue for source algebras, and use this to show that a certain case where this inequality is an equality can be characterised in terms of the structure of the source algebra, generalising a similar result on blocks of minimal dimensions. Let p be a prime and k an algebraically closed field of characteristic p. Let G be a finite group and B a block algebra of kG; that is, B is an indecomposable direct factor of kG as k-algebra. By a result of Brauer in [2], the dimension of B satisfies the inequality dimk(B) ≥ p2a−d · ℓ(B) · u2 B where pa is the order of a Sylow-p-subgroup of G, pd is the order of a defect group of B, ℓ(B) is the number of isomorphism classes of simple B-modules and uB is the unique positive integer such that pa−d · uB is the greatest common divisor of the dimensions of the simple B-modules. It is well-known that uB is prime to p. K¨ulshammer raised the question whether an equality could be expressed in terms of the structure of a source algebra of B, generalising the result in [3] on blocks of minimal dimension. We show that this is the case. The first observation is an analogue for source algebras of Brauer’s inequality. We keep the notation above and refer to [5] for block theoretic background material