Stratifying systems and Jordan-Hölder extriangulated categories

Stratifying systems, which have been defined for module, triangulated and exact categories previously, were developed to produce examples of standardly stratified algebras. A stratifying system Φ is a finite set of objects satisfying some orthogonality conditions. One very interesting property is that the subcategory F(Φ ) of objects admitting a composition series-like filtration with factors in Φ has the Jordan-Hölder property on these filtrations. This article has two main aims. First, we introduce notions of subobjects, simple objects and composition series for an extriangulated category, in order to define a Jordan-Hölder extriangulated category. Moreover, we characterise Jordan-Hölder, length, weakly idempotent complete extriangulated categories in terms of the associated Grothendieck monoid and Grothendieck group. Second, we develop a theory of stratifying systems in extriangulated categories. We define projective stratifying systems and show that every stratifying system Φ in an extriangulated category is part of a minimal projective one (Φ ,Q) . We prove that F(Φ ) is a length, Jordan-Hölder extriangulated category when (Φ ,Q) satisfies a left exactness condition. We give several examples and answer a recent question of Enomoto–Saito in the negative.