The Schur degree of additive sets

Shalom Eliahou (1) , M.P. Revuelta

1 LMPA - Laboratoire de Mathématiques Pures et Appliquées Joseph Liouville

Let (G, +) be an abelian group. A subset of G is sumfree if it contains no elements x, y, z such that x +y = z. We extend this concept by introducing the Schur degree of a subset of G, where Schur degree 1 corresponds to sumfree. The classical inequality S(n) ≤ R n (3) − 2, between the Schur number S(n) and the Ramsey number R n (3) = R(3,. .. , 3), is shown to remain valid in a wider context, involving the Schur degree of certain subsets of G. Recursive upper bounds are known for R n (3) but not for S(n) so far. We formulate a conjecture which, if true, would fill this gap. Indeed, our study of the Schur degree leads us to conjecture S(n) ≤ n(S(n − 1) + 1) for all n ≥ 2. If true, it would yield substantially better upper bounds on the Schur numbers, e.g. S(6) ≤ 966 conjecturally, whereas all is known so far is 536 ≤ S(6) ≤ 1836.

Mots clés: Sumfree; Schur numbers; Ramsey numbers; Discrete derivative; Minors

Domaines: Combinatoire [math.CO]; Théorie des nombres [math.NT]

Shalom Eliahou, M.P. Revuelta. The Schur degree of additive sets. Discrete Mathematics, 2021, 344 (5), pp.112332. ⟨10.1016/j.disc.2021.112332⟩. ⟨hal-02696714⟩