[Maxima-bugs] [ maxima-Bugs-2013650 ] gamma(250.0) returns non-number; gamma(-1.0) finite

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Bugs item #2013650, was opened at 2008-07-08 13:01
Message generated for change (Comment added) made by macrakis
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Category: Lisp Core - Floating point
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Stavros Macrakis (macrakis)
Assigned to: Nobody/Anonymous (nobody)
Summary: gamma(250.0) returns non-number; gamma(-1.0) finite

Initial Comment:
gamma(250.0) and 250.0! return non-numbers. This causes an error in printing ("Can't print a non-number"). It should cause a clean overflow error. The problem is in gamma-lanczos, which doesn't check that its return value is well-defined (not infinite, not a NaN) before returning it.

gamma-lanczos also does not report an error for negative integer arguments: gamma(-1.0) => -2.6e+16

Worse, bfloat(gamma(250.0)) returns a grossly incorrect result -- see next error report.

Tested on Maxima 5.14.0 GCL Windows


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>Comment By: Stavros Macrakis (macrakis)
Date: 2008-07-17 09:51

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As long as we don't have a well-defined policy for Maxima's handling of
IEEE infs and NaNs, it is a bug for any routine to return an IEEE inf or
NaN.  When/if we define the behavior and update the code to work correctly
in the presence of inf/NaN, then it will be fine.

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Comment By: Robert Dodier (robert_dodier)
Date: 2008-07-11 23:41

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I 'm not convinced the problem is in GAMMA-LANCZOS. If the Lisp
implementation permits infinity and NaN as floating point results, then
Maxima functions could just pass those along. The only bug that I see here
for sure is that GCL cannot display NaN and infinity (although such values
can be computed by GCL).

For the sake of uniform behavior, I suppose Maxima functions could reject
NaN and infinity, no matter which Lisp, or allow them. Probably it is
easier to reject NaN and infinity.

I'm undecided at this point about having a uniform policy wrt floating
point special values, or letting it remain an implementation-dependent
feature. 


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