Disjunctive normal form

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"Dnf" redirects here. For other uses, see DNF.

In boolean logic, a disjunctive normal form (DNF) is a standardization (or normalization) of a logical formula which is a disjunction of conjunctive clauses. As a normal form, it is useful in automated theorem proving. A logical formula is considered to be in DNF if and only if it is a disjunction of one or more conjunctions of one or more literals. As in conjunctive normal form (CNF), the only propositional operators in DNF are and, or, and not. The not operator can only be used as part of a literal, which means that it can only precede a propositional variable. For example, all of the following formulas are in DNF:

A \or B
A\!
(A \and B) \or C
(A \and \neg B \and \neg C) \or (\neg D \and E \and F)

However, the following formulas are not in DNF:

\neg(A \or B) — NOT is the outermost operator
A \or (B \and (C \or D)) — an OR is nested within an AND

Converting a formula to DNF involves using logical equivalences, such as the double negative elimination, De Morgan's laws, and the distributive law. Note that all logical formulas can be converted into disjunctive normal form. However, in some cases conversion to DNF can lead to an exponential explosion of the formula. For example, in DNF form, logical formulas of the following form have 2n terms:

(X_1 \or Y_1) \and (X_2 \or Y_2) \and \dots \and (X_n \or Y_n)

The following is a formal grammar for DNF:

  1. <or> → ∨
  2. <and> → ∧
  3. <not> → ¬
  4. <disjunct> → <conjunct>
  5. <disjunct> → <disjunct> <or> <conjunct>
  6. <conjunct> → <literal>
  7. <conjunct> → (<conjunct> <and> <literal>)
  8. <literal> → <term>
  9. <literal> → <not><term>

Where <term> is any variable.

[edit] See also

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