On the idempotent ranks of certain semigroups of order-preserving transformations. Garba, G.U. — 1994 Portugaliae Mathematica
On the nilpotent rank of partial transformation semigroups. Garba, G.U. — 1994 Portugaliae Mathematica
Nilpotents in semigroups of partial one-to-one order-preserving mappings. G.U. Garba — 1994 Semigroup forum