Negation normal form

In mathematical logic, a formula is in negation normal form (NNF) if the negation operator (${\displaystyle \lnot }$, not) is only applied to variables and the only other allowed Boolean operators are conjunction (${\displaystyle \land }$, and) and disjunction (${\displaystyle \lor }$, or).

Negation normal form is not a canonical form: for example, ${\displaystyle a\land (b\lor \lnot c)}$ and ${\displaystyle (a\land b)\lor (a\land \lnot c)}$ are equivalent, and are both in negation normal form.

In classical logic and many modal logics, every formula can be brought into this form by replacing implications and equivalences by their definitions, using De Morgan's laws to push negation inwards, and eliminating double negations. This process can be represented using the following rewrite rules:^[1]

${\displaystyle {\begin{aligned}A\Rightarrow B&~\to ~\lnot A\lor B\\A\Leftrightarrow B&~\to ~(\lnot A\lor B)\land (A\lor \lnot B)\\\lnot (A\lor B)&~\to ~\lnot A\land \lnot B\\\lnot (A\land B)&~\to ~\lnot A\lor \lnot B\\\lnot \lnot A&~\to ~A\\\lnot \exists xA&~\to ~\forall x\lnot A\\\lnot \forall xA&~\to ~\exists x\lnot A\end{aligned}}}$

(In these rules, the ${\displaystyle \Rightarrow }$ symbol indicates logical implication in the formula being rewritten, and ${\displaystyle \to }$ is the rewriting operation.)

Transformation into negation normal form can increase the size of a formula only linearly: the number of occurrences of atomic formulas remains the same, the total number of occurrences of ${\displaystyle \land }$ and ${\displaystyle \lor }$ is unchanged, and the number of occurrences of ${\displaystyle \lnot }$ in the normal form is bounded by the length of the original formula.

A formula in negation normal form can be put into the stronger conjunctive normal form or disjunctive normal form by applying distributivity. Repeated application of distributivity may exponentially increase the size of a formula. In the classical propositional logic, transformation to negation normal form does not impact computational properties: the satisfiability problem continues to be NP-complete, and the validity problem continues to be co-NP-complete. For formulas in conjunctive normal form, the validity problem is solvable in polynomial time, and for formulas in disjunctive normal form, the satisfiability problem is solvable in polynomial time.