(mathematics) A formal power series with one indeterminate, whose coefficients are fractions with factorial denominators (of index corresponding to the power of the indeterminate) and numerators which represent a sequence of numbers that is to be studied.

A double exponential function is a constant raised to the power of an exponential function.

The stretched exponential function is obtained by inserting a fractional power law into the exponential function.

In mathematics, the exponential function can be characterized in many ways.