What is "fo"

Fo \Fo\ (f[=o]), n. The Chinese name of Buddha.

n. (context paper printing English) (abbreviation of folio English), page and book size (10"-12.5" x 15"-20").

Fo or FO may refer to:

In descriptive complexity, a branch of computational complexity, FO is a complexity class of structures which can be recognized by formulas of first-order logic, and also equals the complexity class AC. Descriptive complexity uses the formalism of logic, but does not use several key notions associated with logic such as proof theory or axiomatization.

Restricting predicates to be from a set X yields a smaller class FO[X]. For instance, FO[<] is the set of star-free languages. The two different definitions of FO[<], in terms of logic and in terms of regular expressions, suggests that this class may be mathematically interesting beyond its role in computational complexity, and that methods from both logic and regular expressions may be useful in its study.

Similarly, extensions of FO formed by the addition of operators give rise to other well-known complexity classes. This allows the complexity of some problems to be established without reference to algorithms.