(number theory, analytic number theory, uncountable) The function ζ defined by the Dirichlet series 𝜁(s)=∑ₙ₌₁ ᪲1/(nˢ)=1/(1ˢ)+1/(2ˢ)+1/(3ˢ)+1/(4ˢ)+⋯, which is summable for points s in the complex half-plane with real part > 1; the analytic continuation of said function, being a holomorphic function defined on the complex numbers with pole at 1.

(medicine) any of several biochemical blood assays designed to give information on the state of a patient's liver; include total protein, albumin, alanine transaminase, alkaline phosphatase and bilirubin

(number theory) The function that counts how many integers below a given integer are coprime to it.

(computing) A function key whose function can be assigned by the user or by a program.

(systems theory) That part of a control system that dynamically updates the parameters of an optimizing control function or a direct control function

The transfer function of the feedback path of a control system

The transfer function of the forward path of a control system

(mathematics) Any function in which each point in the domain maps to just one point in the range

(mathematics) A function that is the inverse of an appropriate restriction of a trigonometric function.

(computing) Any of a set of subroutines supplied with a language or system that can be called to perform standard mathematical functions efficiently