Well quasi-orders and context-free grammars

Flavio D'Alessandro, Stefano Varricchio
2004 Theoretical Computer Science  
Let G be a context-free grammar and let L be the language of all the words derived from any variable of G. We prove the following generalization of Higman's theorem: any division order on L is a well quasi-order on L. We also give applications of this result to some quasi-orders associated with unitary grammars.
doi:10.1016/j.tcs.2004.03.069 fatcat:hflig35sjrgb5lebglhbvge4oa