Alessandra Cherubini Pawel Gawrychowski Andrzej Kisielewicz B. Piochi A Combinatorial Approach to Collapsing Words. 256-266 2006 conf/mfcs/2006 MFCS http://dx.doi.org/10.1007/11821069_23 db/conf/mfcs/mfcs2006.html#CherubiniGKP06