Permutations of context-free and indexed languages
  We consider the cyclic closure of a language, and its generalisation to the operators $C^k$ introduced by Brandst\"adt. We prove that the cyclic closure of an indexed language is indexed, and that if $L$ is a context-free language then $C^k(L)$ is indexed. 
Tara Brough, Laura Ciobanu and Murray Elder
cs.FL