Post-plus languages

In this paper, a post-plus language is de.ned as a language which contains all the catenations of each word in the language with a proper su0x of this word. The set of all d-primitive words is a natural post-plus language. The family of all post-plus languages is a subfamily of all non-counting languages. Some basic properties and characterizations of post-plus languages are investigated. We obtain that a post-plus language spanned by a word over an alphabet with two letters is context-free if and only if the language is regular. Some general properties of post-plus languages related to code, dense property and formal language theory such as the nature of context-free, context-sensitive languages are also studied in this paper. c © 2001 Elsevier Science B.V. All rights reserved.