f(x) = (x/b)c-1 * e(-x/b) * [1/b (c)]
0 x, b > 0, c > 0
(gamma) is the Gamma function
b is the scale parameter
a is the so-called shape parameter
e is the base of the natural logarithm, sometimes called Euler's e (2.71...)
The animation above shows the gamma distribution as the shape parameter changes from 1 to 6.