Résumé
Given a set
I
of words, the set
L
⊢
I
ϵ
of all words obtained by the shuffle of (copies of) words of
I
is naturally provided with a partial order: for
u
,
v
in
L
⊢
I
ϵ
,
u
⊢
I
∗
v
if and only if
v
is the shuffle of
u
and another word of
L
⊢
I
ϵ
. In [F. D’Alessandro, S. Varricchio, Well quasi-orders, unavoidable sets and derivation systems, in: Word Avoidability Complexity and Morphisms (WACAM), RAIRO Theoretical Informatics and Applications 40 (3) (2006) 407–426 (special issue)], the authors have opened the problem of the characterization of the finite sets
I
such that
⊢
I
∗
is a well quasi-order on
L
⊢
I
ϵ
. In this paper we give an answer in the case when
I
consists of a single word
w
.