In mathematics, several functions are important enough to deserve their own name. This is a listing of pointers to those articles which explain these functions in more detail.
Polynomials: can be generated by addition and multiplication alone.
Square root: yields a number whose square is the given one.
Exponential function: raise a fixed number to a variable power.
Logarithm: the inverses of exponential functions; useful to solve equations involving exponentials.
Trigonometric functions: sine, cosine, etc.; used in geometry and to describe periodic phenomena.
Hyperbolic functions: formally similar to the trigonometric functions.
Absolute value: drops the sign of a given number.
Floor function: largest integer ≤ a given number.
Gamma function: A generalization of the factorial function.
Riemann zeta function: A special case of Dirichlet series.
Elliptic integrals: Arising from the path length of ellipses; important in many applications.
Elliptic functions: The inverses of elliptic integrals; used to model double-periodic phenomena.
Bessel functions: Defined by a differential equation; useful in astronomy, electromagnetism, and mechanics.
Logarithmic integral: Integral of the reciprocal of the logarithm, important in the prime number theorem.
Lambert's W function: inverse of f(w) = w exp(w).
Error function: an integral important for normal random variables.
Number theoretic functions
sigma function: Sums of powers of factors of a given natural number.
Euler's phi function: Number of numbers relatively prime to a given one.
Prime counting function: Number of primess less than or equal to a given number.
Partition function: Order-independent count of ways to write a given positive integer as a sum of positive integers.
Ackermann function: in the theory of computation, a recursive function that is not primitive recursive.
Dirac delta function: everywhere zero except for x = 0; total integral is 1. Not a function but a distribution.
Heaviside step function: 0 for negative arguments and 1 for positive arguments. The integral of the Dirac delta distribution.