# Literal (mathematical logic)

In mathematical logic, a **literal** is an atomic formula (atom) or its negation.
The definition mostly appears in proof theory (of classical logic), e.g. in conjunctive normal form and the method of resolution.

Literals can be divided into two types:

- A
**positive literal**is just an atom (e.g., ). - A
**negative literal**is the negation of an atom (e.g., ).

The **polarity** of a literal is positive or negative depending on whether it is a positive or negative literal.

For a literal , the **complementary literal** is a literal corresponding to the negation of ,
we can write to denote the complementary literal of . More precisely, if then is and if then is .

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.

In Boolean functions, the variables that appear either in complemented or uncompleted form is a literal. For example, if , and are variables then the expressions contains three literals and contains three literals.[1]

## Examples

In propositional calculus a literal is simply a propositional variable or its negation.

In predicate calculus a literal is an atomic formula or its negation, where an atomic formula is a predicate symbol applied to some terms, with the terms recursively defined starting from constant symbols, variable symbols, and function symbols. For example, is a negative literal with the constant symbol 2, the variable symbols *x*, *y*, the function symbols *f*, *g*, and the predicate symbol *Q*.

## References

- A. P. Godse, D. A. Godse (2008).
*Digital Logic Circuits*. Technical Publications. ISBN 9788184314250.

- Samuel R. Buss (1998). "An introduction to proof theory". In Samuel R. Buss (ed.).
*Handbook of proof theory*. Elsevier. pp. 1–78. ISBN 0-444-89840-9.