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Atomic formula : ウィキペディア英語版
Atomic formula
In mathematical logic, an atomic formula (also known simply as an atom) is a formula with no deeper propositional structure, that is, a formula that contains no logical connectives or equivalently a formula that has no strict subformulas. Atoms are thus the simplest well-formed formulas of the logic. Compound formulas are formed by combining the atomic formulas using the logical connectives.
The precise form of atomic formulas depends on the logic under consideration; for propositional logic, for example, the atomic formulas are the propositional variables. For predicate logic, the atoms are predicate symbols together with their arguments, each argument being a term. In model theory, atomic formula are merely strings of symbols with a given signature, which may or may not be satisfiable with respect to a given model.
==Atomic formula in first-order logic==
The well-formed terms and propositions of ordinary first-order logic have the following syntax:
Terms:
* t \equiv c \mid x \mid f (t_, ..., t_),
that is, a term is recursively defined to be a constant ''c'' (a named object from the domain of discourse), or a variable ''x'' (ranging over the objects in the domain of discourse), or an ''n''-ary function ''f'' whose arguments are terms ''t''''k''. Functions map tuples of objects to objects.
Propositions:
* A, B, ... \equiv P (t_, ..., t_) \mid A \wedge B \mid \top \mid A \vee B \mid \bot \mid A \supset B \mid \forall x.\ A \mid \exists x.\ A ,
that is, a proposition is recursively defined to be an ''n''-ary predicate ''P'' whose arguments are terms ''t''''k'', or an expression composed of logical connectives (and, or) and quantifiers (for-all, there-exists) used with other propositions.
An atomic formula or atom is simply a predicate applied to a tuple of terms; that is, an atomic formula is a formula of the form ''P'' (''t''1, …, ''t''''n'') for ''P'' a predicate, and the ''t''''n'' terms.
All other well-formed formulae are obtained by composing atoms with logical connectives and quantifiers.
For example, the formula ∀''x. P'' (''x'') ∧ ∃''y. Q'' (''y'', ''f'' (''x'')) ∨ ∃''z. R'' (''z'') contains the atoms
* P (x)
* Q (y, f (x))
* R (z)
When all of the terms in an atom are ground terms, then the atom is called a ground atom or ''ground predicate''.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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