Propositional Logic Predicate Logic
... Definition. A formula A is valid if A is true no matter how we replace the individual constants
in A with concrete individuals and the predicate variables in A with concrete predicates.
Note. The set of individuals must be instantiated to a non-empty set. This the reason why
(∀x.P (x)) ⇒ (∃x.P (x)) ...
... where the Ai can be either atomic formulas or their negations. It is thus a conjuction of clauses, i.e. disjunctions of literals (variables or their negations).
CNF is used in machine proving of theorems. Resolution in Prolog is also
based on a special form of CNF.
Definition 5.10 (Disjunctive norma ...
... Let be an unsatisfiable formula
Theorem 2: A proof P is complete with respect to if
for every full assignment
Boolean satisfiability problem
In computer science, the Boolean Satisfiability Problem (sometimes called Propositional Satisfiability Problem and abbreviated as SATISFIABILITY or SAT) is the problem of determining if there exists an interpretation that satisfies a given Boolean formula. In other words, it asks whether the variables of a given Boolean formula can be consistently replaced by the values TRUE or FALSE in such a way that the formula evaluates to TRUE. If this is the case, the formula is called satisfiable. On the other hand, if no such assignment exists, the function expressed by the formula is identically FALSE for all possible variable assignments and the formula is unsatisfiable. For example, the formula ""a AND NOT b"" is satisfiable because one can find the values a = TRUE and b = FALSE, which make (a AND NOT b) = TRUE. In contrast, ""a AND NOT a"" is unsatisfiable.SAT is one of the first problems that was proven to be NP-complete. This means that all problems in the complexity class NP, which includes a wide range of natural decision and optimization problems, are at most as difficult to solve as SAT. There is no known algorithm that efficiently solves SAT, and it is generally believed that no such algorithm exists; yet this belief has not been proven mathematically, and resolving the question whether SAT has an efficient algorithm is equivalent to the P versus NP problem, which is the most famous open problem in the theory of computing.Despite the fact that no algorithms are known that solve SAT efficiently, correctly, and for all possible input instances, many instances of SAT that occur in practice, such as in artificial intelligence, circuit design and automatic theorem proving, can actually be solved rather efficiently using heuristical SAT-solvers. Such algorithms are not believed to be efficient on all SAT instances, but experimentally these algorithms tend to work well for many practical applications.