a^b when a is large and b < 1 (64bit R on windows 7)

Hi,

The problem here appears to be due to 

                          29: #define MAXNUM 1.7976931348623158E308

(from
http://cygwin.com/cgi-bin/cvsweb.cgi/src/winsup/mingw/mingwex/math/cephe
s_mconf.h?annotate=1.5&cvsroot=src)

and

                         468: if( x >= MAXNUM )
                         469:        {
                         470: #if INFINITIES
                         471:        if( y > 0.0 )
                         472:                return( INFINITY );
(from
http://cygwin.com/cgi-bin/cvsweb.cgi/src/winsup/mingw/mingwex/math/pow.c
?annotate=1.1.10.1&cvsroot=src)

And so on 64bit Windows R build
.Machine$double.xmax = MAXMUM from above.

This is kind of confirmed as 

(.Machine$double.xmax*(1 - .Machine$double.eps))^0.5

works as expected 

Martyn

-----Original Message-----
From: r-devel-bounces at r-project.org
[mailto:r-devel-bounces at r-project.org] On Behalf Of Bryan W. Lewis
Sent: 21 November 2011 23:47
To: Joris Meys
Cc: r-devel at r-project.org
Subject: Re: [Rd] a^b when a is large and b < 1 (64bit R on windows 7)

Although it does not solve the issue, the INF result is due to the
MinGW gcc math library on 64-bit Windows that R links to. One can
easily see this by compiling just machar.c and checking pow and sqrt
on xmax.

Note that on any platform the results are likely to be quite different
between pow and sqrt for edge case numerical examples.

For example, on a 32-bit Ubuntu box with R-2.14.0, I get:
x = .Machine$double.xmax
y = sqrt(x)
z = x^0.5
y-z
[1] 1.488566e+138

There is also a long discussion about this here:

http://sourceforge.net/mailarchive/forum.php?forum_name=mingw-users&max_
rows=25&style=nested&viewmonth=201104

--Bryan

On Mon, Nov 21, 2011 at 10:03 AM, Joris Meys <jorismeys at gmail.com>
wrote:

a^b when a is large and b < 1 (64bit R on windows 7)

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