# What is the history of moment generating functions, and the more general characteristic functions?

Moment generating functions provide an alternative specification for a probability distribution function (pdf), often making it very convenient to calculate expectations, variances, etc. of said pdf.

A moment generating function is defined as:

$M_X(t) = \mathbb{E}(e^{tX}), t\in\mathbb{R}$

A caveat to moment generating functions is that their integrals might not always exist. The more general characteristic function gets around this, and they are defined as:

$C_X(t) = \mathbb{E}(e^{itX}), t\in\mathbb{R}$

Given their tremendous capacity for simplifying the calculation of the moments of a probability distrubtion, moment-generating and characteristic functions seem almost magical -- a wonderful rabbit pulled, from thin air, out of a magician's hat.

Few things are truly that way in the history of mathematics, so I am curious to know how this particular rabbit was born: how did human beings zero in on them?

The general idea of generating function has much wider scope than its applications to probability. The proper setting is harmonic analysis'' which is one of the central and most developed parts of mathematics. The birth of the idea can be traced back to Abraham de Moivre (1667-1754), and his book Doctrine of Chances. Later the same idea was developed and applied in number theory (Euler), and most importantly in mathematical physics (Fourier). (Characteristic function is a special case of Fourier transform). Laplace transform (the moment generating function) belongs to the same circle of ideas, and its original use was also in probability.