Conversion To Clausal Normal Form
The procedure to convert a formula into clausal form can destroy the structure of the formula, and naive translations often causes exponential blowup in the size of the resulting formula.
The procedure begins with any formula of classical first-order logic:
- Put the formula into negation normal form.
- Standardize variables
- becomes, where is new
- Skolemize -- replace existential variables with Skolem constants or Skolem functions of universal variables, from the outside inward. Make the following replacements:
- becomes, where is new
- Discard the universal quantifiers (which are implicit in CNF).
- Put the formula into conjunctive normal form.
- Replace with . Each conjunct is of the form, which is equivalent to .
When n ≤ 1 for all clauses, the logic is called Horn clause logic and is equivalent in computational power to a universal Turing machine. Horn logic is the basis of Prolog, the most widely used logic programming language.
Often it is sufficient to generate an equisatisfiable (not an equivalent) CNF for a formula. In this case, the worst-case exponential blow-up can be avoided by introducing definitions and using them to rename parts of the formula.
Read more about this topic: Clausal Normal Form
Famous quotes containing the words conversion, normal and/or form:
“The conversion of a savage to Christianity is the conversion of Christianity to savagery.”
—George Bernard Shaw (18561950)
“Separation anxiety is normal part of development, but individual reactions are partly explained by experience, that is, by how frequently children have been left in the care of others.... A mother who is never apart from her young child may be saying to him or her subliminally: You are only safe when Im with you.”
—Cathy Rindner Tempelsman (20th century)
“I am afraid I am one of those people who continues to read in the hope of sometime discovering in a book a singleand singularpiece of wisdom so penetrating, so soul stirring, so utterly applicable to my own life as to make all the bad books I have read seem well worth the countless hours spent on them. My guess is that this wisdom, if it ever arrives, will do so in the form of a generalization.”
—Joseph Epstein (b. 1937)