# Gamma

Gamma-func

## Definition:

evaluates

The function is based on a Chebyshev expansion for (1+u), and uses the property (1+x) = x(x).

If x = N +1+u where N is integral and 0 ≤ u < 1 then it follows that:

for N >0 (x) = (x - 1)(x - 2) . . . (x - N)(1 + u)

for N = 0 (x) = (1+u)

for N <0 (x) = (1+u)/x(x + 1)(x + 2) . . . (x - N - 1).

For more information please review the s14aac function in the NAG document

## Parameters:

- x (input, double)
- The argument x of the function.
- Constraint: x must not be zero or a negative integer.
- (output, double)
- the value of the function