NAG C Library Manual, Mark 26
NAG Library Manual, Mark 26
NAG AD Library Manual, Mark 26
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NAG C Library Function Document

nag_real_polygamma (s14aec)

NAG C Library Manual, Mark 26
NAG Library Manual, Mark 26
NAG AD Library Manual, Mark 26
s Chapter Contents
s Chapter Introduction

▸▿ Contents

© The Numerical Algorithms Group Ltd. 2018

1
Purpose

nag_real_polygamma (s14aec) returns the value of the kth derivative of the psi function ψx for real x and k=0,1,…,6.

2
Specification

#include <nag.h>
#include <nags.h>
double  nag_real_polygamma (double x, Integer k, NagError *fail)

3
Description

nag_real_polygamma (s14aec) evaluates an approximation to the kth derivative of the psi function ψx given by
ψ k x=dkdxk ψx=dkdxk ddx loge⁡Γx ,  
where x is real with x≠0,-1,-2,… and k=0,1,…,6. For negative noninteger values of x, the recurrence relationship
ψ k x+1=ψ k x+dkdxk 1x  
is used. The value of -1k+1ψ k x k!  is obtained by a call to nag_polygamma_deriv (s14adc), which is based on the function PSIFN in Amos (1983).
Note that ψ k x is also known as the polygamma function. Specifically, ψ 0 x is often referred to as the digamma function and ψ 1 x as the trigamma function in the literature. Further details can be found in Abramowitz and Stegun (1972).

4
References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
Amos D E (1983) Algorithm 610: A portable FORTRAN subroutine for derivatives of the psi function ACM Trans. Math. Software 9 494–502

5
Arguments

1:     x – doubleInput
On entry: the argument x of the function.
Constraint: x must not be ‘too close’ (see Section 6) to a non-positive integer.
2:     k – IntegerInput
On entry: the function ψkx to be evaluated.
Constraint: 0≤k≤6.
3:     fail – NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

6
Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 2.3.1.2 in How to Use the NAG Library and its Documentation for further information.
NE_INT
On entry, k=value.
Constraint: k≤6.
On entry, k=value.
Constraint: k≥0.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
NE_OVERFLOW_LIKELY
Evaluation abandoned due to likelihood of overflow.
NE_REAL
On entry, x is ‘too close’ to a non-positive integer: x=value and nintx=value.
NE_UNDERFLOW_LIKELY
Evaluation abandoned due to likelihood of underflow.

7
Accuracy

All constants in nag_polygamma_deriv (s14adc) are given to approximately 18 digits of precision. If t denotes the number of digits of precision in the floating-point arithmetic being used, then clearly the maximum number in the results obtained is limited by p=mint,18. Empirical tests by Amos (1983) have shown that the maximum relative error is a loss of approximately two decimal places of precision. Further tests with the function -ψ 0 x have shown somewhat improved accuracy, except at points near the positive zero of ψ 0 x at x=1.46…, where only absolute accuracy can be obtained.

8
Parallelism and Performance

nag_real_polygamma (s14aec) is not threaded in any implementation.

9
Further Comments

None.

10
Example

This example evaluates ψ 2 x at x=2.5, and prints the results.

10.1
Program Text

Program Text (s14aece.c)

10.2
Program Data

Program Data (s14aece.d)

10.3
Program Results

Program Results (s14aece.r)

NAG C Library Manual, Mark 26
NAG Library Manual, Mark 26
NAG AD Library Manual, Mark 26
s Chapter Contents
s Chapter Introduction
© The Numerical Algorithms Group Ltd. 2018