Remarks on partially abelian exact categories

Abstract: The purpose of this short and elementary note is to identify some classes of exact categories introduced in L. Previdi's thesis. Among other things we show: (1) An exact category is partially abelian exact if and only if it is abelian. (2) An exact category satisfies the axioms AIC and AIC{\deg} if and only if it is quasi-abelian in the sense of J.-P. Schneiders. (3) An exact category satisfies AIC if and only if it is an additive category of the type considered by G. Laumon in his work on derived categories of filtered ${\cal D}$-modules. In all of the above classes all morphisms have kernels and coimages and the exact structure must be given by all kernel-cokernel pairs.