Well quasi-orders and the shuffle closure of finite sets
  Given a set I of word, the set of all words obtained by the shuffle of (copies of) words of I is naturally provided with a partial order. In [FS05], the authors have opened the problem of the characterization of the finite sets I such that the order is a well quasi-order . In this paper we give an answer in the case when I consists of a single word w. 
Flavio D'Alessandro, Gw\'ena\"el Richomme (LaRIA), Stefano Varrichio