Semigroups of order-preserving partial endomorphisms on trees, I Carlton J. Maxson — 1974 Colloquium Mathematicae
A matrix characterization of the maximal groups in β X Darald J. Hartfiel; Carlton J. Maxson — 1975 Czechoslovak Mathematical Journal
Lattice endomorphisms of 2 X Carlton J. Maxson; Ponnammal Natarajan — 1977 Czechoslovak Mathematical Journal