(mathematics) A function whose value is always the same as its independent variable, and for which the codomain equals the domain.

(mathematics) Any function that is a solution of the hypergeometric equation; usually an expansion of an hypergeometric series

(mathematics) For a given function f, another function, denoted f⁻¹, that reverses the mapping action of f; (formally) given a function f:X→Y, a function g:Y→X such that, ∀x∈X,f(x)=y⟹g(y)=x.

(mathematics) Any function, in the calculus of variations, that satisfies the Euler equations

(mathematics) Any function of a real variable whose value increases (or is constant) as the variable increases.

(mathematics) A printed tabulation of selected values of a function corresponding to selected independent variables; e.g. a table of logarithms.

(mathematics) A function that assigns a pair of vertices to each edge of a graph.

(mathematics) Any generalized function which is defined by its behavior under integration.

(management) Any primary business function that would affect customers or profits if interrupted

(electronics) An electronic logic circuit which inverts the phase of a signal