1. 1987 Boundly factorizable $\mathcal{S}$-indecomposable semigroups generated by two elements

2. He solved the problem of finding which abelian groups have a finite number of indecomposable modules.

3. A module of finite length is indecomposable if and only if its endomorphism ring is local.

4. This included decomposition numbers, Ext-quivers, indecomposable modules, Cartan matrices and properties of gradings related to the relevant algebras.

5. He was an Invited Speaker at the International Congress of Mathematicians in 1983 in Warsaw with talk Indecomposable representations of finite-dimensional algebras.