Recursively Enumerable Languages
- This chapter shows that the family of regular languages is a proper subset of context free languages and the latter is a proper subset of recursively enumerable languages.
- http://www.cs.duke.edu/courses/cps140/spring03/lects/sectRecEnumH.pdf#search=%22recursively%20enumerable%20languages%22

Recursively Enumerable Language
- Wikipedia offers 3 equivalent definitions of recursively enumerable languages and states closure properties for certain operations.
- http://en.wikipedia.org/wiki/Recursively_enumerable_language

Turing Machines and Type 0 Languages
- Two theorems which show that the class of languages accepted by Turing machines is the class of Type 0 or recursively enumerable languages.
- http://www.cse.ohio-state.edu/~gurari/theory-bk/theory-bk-fourse6.html

Phrase Structure Grammars and Context Sensitive Grammars
- A site which shows that the languages generated by phrase structure grammars are recursively enumerable languages. Similarly, it shows that the languages generated by context sensitive grammars are accepted by linear bounded automata.
- http://www.cis.upenn.edu/~cis511/Spring04/cis51104sl13pdf.pdf#search=%22context%20sensitive%20languages%22