Pattern Avoidability with Involution
  An infinte word w avoids a pattern p with the involution t if there is no substitution for the variables in p and no involution t such that the resulting word is a factor of w. We investigate the avoidance of patterns with respect to the size of the alphabet. For example, it is shown that the pattern a t(a) a can be avoided over three letters but not two letters, whereas it is well known that a a a is avoidable over two letters. 
Bastian Bischoff (Institute for Formal Methods in Computer Science,   Universit\"at Stuttgart), Dirk Nowotka (Institute for ...
cs.FL cs.DM math.CO