Evaluates the derivative of the log gamma function.
Synopsis
#include <imsl.h>
float imsl_f_psi (float x)
The type double function is imsl_d_psi.
Required Arguments
float x
(Input)
Argument at which the function is to be evaluated.
Return Values
The value of the derivative of the log gamma function at x. NaN is returned if an error occurs.
Description
The psi function, also called the digamma function, is defined to be
See imsl_f_gamma for the definition of Γ(x).
The argument x
must not be exactly zero or a negative integer, or 
is undefined. Also, x
must not be too close to a negative integer such that the accuracy of the result
is less than half precision. If no value can be computed, then NaN is
returned.
Example
In this example, 
is evaluated.
#include <imsl.h>
#include <stdio.h>
int main(){
float x=1.915, ans;
ans=imsl_f_psi(x);
printf("psi(%f) = %f\n", x, ans);
}
Output
psi(1.915000) = 0.366452
Warning Errors
IMSL_NEAR_NEG_INT_WARN The result is accurate to less than one-half precision because “x” is too close to a negative integer.
