Evaluates the second derivative of the log gamma function.
Synopsis
#include <imsl.h>
float imsls_f_psi1 (float x)
The type double function is imsl_d_psi1.
Required Arguments
float x
(Input)
Argument at which the function is to be evaluated.
Return Value
The value of the second derivative of the log gamma function at x. NaN is returned if an error occurs.
Description
The psi1 function, also called the trigamma 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.
Example
In this example, 
is evaluated.
#include <imsl.h>
#include <stdio.h>
int main(){
float x=1.915, ans;
ans=imsl_f_psi1(x);
printf("psi1(%f) = %f\n", x, ans);
}
Output
psi1(1.915000) = 0.681164
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.
