On a Constant Word in Finite Semigroups. - Brunetto Piochi; Giuseppe Pirillo (Eur. J. Comb., 1989)

A Combinatorial Approach to Collapsing Words. - Alessandra Cherubini; Pawel Gawrychowski; Andrzej Kisielewicz 001; Brunetto Piochi (MFCS, 2006)

On the length of shortest 2-collapsing words. - Alessandra Cherubini; Andrzej Kisielewicz 001; Brunetto Piochi (Discret. Math. Theor. Comput. Sci., 2009)

Short 3-Collapsing Words over a 2-Letter Alphabet. - Alessandra Cherubini; Achille Frigeri; Brunetto Piochi (Developments in Language Theory, 2011)