Two papers: \(\mathcal H\)-coextensions of monoids and The structure of a band of groups. (English) Zbl 1415.20002
Memoirs of the American Mathematical Society 157. Providence, RI: American Mathematical Society (AMS) (ISBN 978-0-8218-1857-2; 978-1-4704-0543-4/ebook). 95 p. (1975).