1. Åqvist, Lennart: 1965, A New Approach to the Logical Theory of Interrogatives, Filosofiska Föreningen. Uppsala, Sweden.
2. Bach, Emmon: 1971, ?Questions?, Linguistic Inquiry 2, 153?166.
3. Baker, Carl L.: 1968, Indirect Questions in English, Ph.D. dissertation. University of Illinois. Urbana, Ill.
4. Baker, Carl L.: 1970, ?Notes on the Description of English Questions: The Role of an Abstract Question, Morpheme?, Foundations of Language 6, 197?217.
5. Technical Memorandum TM 1287/00/000;Nuel Belnap,1963