A. Pietrowski

An Improved Subgroup Theorem for HNN Groups with Some Applications

A. Karrass, D. Solitar, A. Pietrowski, 1974, Canadian Journal of Mathematics.

On a Conjecture of W. Magnus

A. Pietrowski, J. McCool, 1973 .

Presentations of the Trefoil Group

M. J. Dunwoody, A. Pietrowski, 1973, Canadian Mathematical Bulletin.

Finite and infinite cyclic extensions of free groups

A. Karrass, D. Solitar, A. Karrass, 1973, Journal of the Australian Mathematical Society.