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Re: Math Question [message #51118 is a reply to message #51117]

Sun, 29 October 2006 09:59

Foldy Lajos
Messages: 268
Registered: October 2001

Senior Member

On Sun, 29 Oct 2006, David Fanning wrote:

> Folks,
>
> OK, I get the feeling that I am going to be referred to
> my own web page with this question:
>
> http://www.dfanning.com/math_tips/sky_is_falling.html
>
> And it is certainly true that I have been watching WAY
> too much TV lately (World Series, you know), but here is
> my question. How does one explain the following two
> IDL commands?
>
> IDL> Help, (-0.1)^2.0
> <Expression> FLOAT = 0.0100000
> IDL> Help, (-0.1)^2.01
> <Expression> FLOAT = -NaN
>
> In general, raising a negative number to any integer
> power seems to produce a real number, whereas raising
> a negative number to a non-integer power causes a NAN.
>
> I am sure this is explained in one of those textbooks
> covered with dust in my garage, but I thought one of
> you math guys could rescue me from a beautiful day
> spent covered with dust. :-)
>
> Cheers,
>
> David
> --
> David Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Coyote's Guide to IDL Programming: http://www.dfanning.com/
> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
>

I had the opposite problem with FL: (-0.1)^2.0 was also a NaN. Power is
defined for floats as x^y = exp(log(x)*y), and log() is undefined for
non-positive numbers. Now FL examines y first, and if its is an integer,
then uses multiplications. I think IDL does the same.

regards,
lajos

ps: what will be the result for (-0.1)^2.0000001 ? :-)

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[Message index]

		Re: Math Question By: rkombiyil on Wed, 01 November 2006 04:51
		Re: Math Question By: Jo Klein on Tue, 31 October 2006 04:33
		Re: Math Question By: news.qwest.net on Mon, 30 October 2006 13:07
		Re: Math Question By: Jean H. on Mon, 30 October 2006 12:32
		Re: Math Question By: Jean H. on Mon, 30 October 2006 12:35
		Re: Math Question By: Jo Klein on Mon, 30 October 2006 12:17
		Re: Math Question By: Braedley on Mon, 30 October 2006 10:22
		Re: Math Question By: James Kuyper on Mon, 30 October 2006 10:15
		Re: Math Question By: David Fanning on Mon, 30 October 2006 09:19
		Re: Math Question By: James Kuyper on Mon, 30 October 2006 09:04
		Re: Math Question By: news.qwest.net on Mon, 30 October 2006 08:54
		Re: Math Question By: Foldy Lajos on Mon, 30 October 2006 07:29
		Re: Math Question By: edward.s.meinel@aero. on Mon, 30 October 2006 07:19
		Re: Math Question By: Rob Dimeo on Sun, 29 October 2006 16:15
		Re: Math Question By: Robbie on Sun, 29 October 2006 15:40
		Re: Math Question By: Robbie on Sun, 29 October 2006 15:01
		Re: Math Question By: Foldy Lajos on Sun, 29 October 2006 09:59

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