Issue 0244: tgamma(zero or negative integer) should be considered a pole error

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Fred Tydeman (US)
Date: 2001-02-25
Reference document: ISO/IEC WG14 N943
Submitted against: C99
Status: Fixed
Fixed in: C99 TC2
Converted from: summary-c99.htm, dr_244.htm

Summary

tgamma(zero or negative integer)should be considered a pole error since the correct mathematical result is an exact infinity (whose sign depends upon the side that the limit is taken from). Annex F already does this for the zero argument case.

Details

When the correct mathematical result is an exact infinity (from finite arguments), it is considered a pole or singularity error. This is true if the result's sign is independent of the direction of the limit. It is believed to be true that it still is a pole error if the sign of the infinite result depends upon the direction of the limit.

Some symbolic math packages support the concept of +/-infinity in addition to +infinity and -infinity, and in those, tgamma(zero or negative integer) is +/-infinity.

Since +/-infinity cannot be represented in most (all?) floating-point formats, return +infinity for that value.

LIA-2 treats similar cases (math function with exact non-zero integer argument and a result of +/-infinity, such as tan(90 degrees)) as a pole error with the result of signed infinity.

Counter-argument: For 0.0 we have the luxury of +0.0 and -0.0. Non-zero integers don't have "sides". There is no concept of +/-infinity in IEC 60559 (nor any other hardware floating-point representation), just +infinity and -infinity. If there is no one correct result for a given argument, then that case must be considered an invalid operation or a domain error. tgamma(negative integer) has two results (+infinity or -infinity), so must be considered invalid. tgamma(x), as x approaches -infinity, has no unique limit, so must also be considered invalid.

Suggested Technical Corrigendum

In 7.12.8.4 tgamma:

Change:

A domain error occurs if x is a negative integer or if the result cannot be represented when x is zero.

to

A range error may occur if x is a negative integer or zero.

In F.9.5.4 tgamma:

Change:

tgamma(x) returns a NaN and raises the "invalid" floating-point exception for x a negative integer.

to

tgamma(x) returns +INF and raises the "divide-by-zero" floating-point exception for x a negative integer.

Change:

tgamma(-INF) returns a NaN and raises the "invalid" floating-point exception.

to

tgamma(-INF) returns +INF and raises the "divide-by-zero" floating-point exception.

Comment from WG14 on 2001-10-16:

Technical Corrigendum

In 7.12.8.4 tgamma:

Change:

A domain error occurs if x is a negative integer or if the result cannot be represented when x is zero.

to

A domain error or range error may occur if x is a negative integer or zero.