In mathematical logic, a literal is an atomic formula (atom) or its negation. They mostly appear in the context of conjunctive normal form and the method of resolution.
Literals can be divided into two types:
- A positive literal is just an atom.
- A negative literal is the negation of an atom.
For a literal l, the complementary literal is a literal corresponding to the negation of l, we can write 
to denote the complementary literal of l. More precisely, if 
then is 
and if 
then is x.
In the context of a formula in the conjunctive normal form, a literal is pure if the literal's complement does not appear in the formula.
References
- Buss, Samuel (1998). "An introduction to proof theory". Handbook of proof theory. Elsevier. pp. 1–78. ISBN 0-444-89840-9. http://math.ucsd.edu/~sbuss/ResearchWeb/handbookI/.
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