> < ^ Date: Tue, 26 Jan 1993 17:40:11 +0100
> < ^ From: Martin Schoenert <martin.schoenert@math.rwth-aachen.de >
< ^ Subject: Re: FactorsInt and 6th Fermat Number
In his e-mail message of 1993/01/26 Edward Spitznagel wrote:

I tried factoring the 6th Fermat number using the code below and
got F6 returned to me, suggesting that F6 is prime. Since one
factor of F6 is 274177, this seems to contradict Section 10.20
of the documentation, which states that "FactorsInt is
guaranteed to find all factors less than 10^6."

gap> FactorsInt( 2^(2^6) + 1);
[ 18446744073709551617 ]
gap> (2^(2^6) + 1) / 274177;
gap> quit;

This is indeed a bug in the function 'IsPrime'. Since 'IsPrime(2^64+1)'
returns 'true', 'Factors' doesn't even attempt to factor it. A fix for
that bug is in the upgrade from GAP 3.1 patchlevel 2 to GAP 3.1
patchlevel 3, which is available as 'upg3r1p3.dif.Z' from

More about the problem (i.e., the reason why 'IsPrime' failed), can be
found in one of my e-mails to the GAP Forum, which is in the file
'forum92d.txt' (GAP Forum mails of 4th quarter 92), on
'samson.math.rwth-aachen.de' in the directory 'tmp' (not in 'pub/gap' as
the rest of GAP).

In his e-mail message of 1993/01/26 Ansgar Kaup answered

I tried factoring the 6th Fermat number in order to check the result that
Edward Spitznagel got with his NeXT and FactorsInt found the correct factors.
I am using a SUN SPARC station 10 and got the executable from SAMSON aswell.
So the bug must have something to do with Edwards Hardware or is contained in
the executable obtained from SAMSON.

I think the NeXT people would be quite offended by the idea that their
hardware is to blame ;-). (Oh, and of course the executables we
distribute never have bugs. Well, hardly ever ;-)


-- .- .-. - .. -.  .-.. --- ...- . ...  .- -. -. .. -.- .-
Martin Sch"onert,   Martin.Schoenert@Math.RWTH-Aachen.DE,  +49 241 804551
Lehrstuhl D f"ur Mathematik, Templergraben 64, RWTH, D 51 Aachen, Germany

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