Let us consider an additive category 𝒜 with arbitrary direct sums (also called coproducts).
Definition 0.1. An object X of 𝒜 is called compact if, for an arbitrary set of objects of 𝒜
and a morphism
there exists some finite set S ⊂ I such that Im f is a subobject of α ∈⊕
α∈SMα.