A cancellative hypergraph has no three edges A;B;C with ADBCC: We give a new short proof of an old result of Bollobás, which states that the maximum size of a cancellative triple system is achieved by the balanced complete tripartite 3-graph. One of the two forbidden subhypergraphs in a cancellative 3-graph is F5 1⁄4 fabc; abd; cdeg: For nX33 we show that the maximum number of triples on n vert...

Problems involving chains of irreducible factorizations in atomic integral domains and monoids have been the focus of much recent literature. If S is a commutative cancellative atomic monoid, then the catenary degree of S (denoted c(S)) and the tame degree of S (denoted t(S)) are combinatorial invariants of S which describe the behavior of chains of factorizations. In this note, we describe met...

This paper applies the concept of FA-presentable structures to semigroups. We give a complete classification of the finitely generated FA-presentable cancellative semigroups: namely, a finitely generated cancellative semigroup is FA-presentable if and only if it is a subsemigroup of a virtually abelian group. We prove that all finitely generated commutative semigroups are FA-presentable. We giv...

Every cancellative Kleene semigroup satisfies Eilenberg's theorem. Résumé. — Si S est un semigroupe simplifiable de type Kleene, alors S satisfait le théorème d'Eilenberg.

Cancellative dimer algebras on a torus have many nice algebraic and homological properties. However, these properties disappear for higher genus surfaces. We consider new class of quiver surfaces, called 'geodesic ghor algebras', that reduce to cancellative torus, yet continue These exhibit rich interplay between their central geometry the topology surface. show (nontrivial) geodesic do in fact...

The following problem is considered: when can the action of a cancellative semigroup S on a set be extended to a simply transitive action of the universal group of S on a larger set.