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From "Thomas Neidhart (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MATH-904) FastMath.pow deviates from Math.pow for negative, finite base values with an exponent 2^52 < y < 2^53
Date Wed, 21 Nov 2012 18:43:57 GMT

    [ https://issues.apache.org/jira/browse/MATH-904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13502188#comment-13502188
] 

Thomas Neidhart commented on MATH-904:
--------------------------------------

Does anyone know the reason why numbers > 2^53 are assumed to be even?

Looking at the javadoc of Math.pow, this is not mentioned, only the general case:

{noformat}
If the first argument is finite and less than zero
    if the second argument is a finite even integer, the result is equal to the result of
raising the absolute value of the first argument to the power of the second argument
    if the second argument is a finite odd integer, the result is equal to the negative of
the result of raising the absolute value of the first argument to the power of the second
argument
    if the second argument is finite and not an integer, then the result is NaN. 
{noformat}
                
> FastMath.pow deviates from Math.pow for negative, finite base values with an exponent
2^52 < y < 2^53 
> ------------------------------------------------------------------------------------------------------
>
>                 Key: MATH-904
>                 URL: https://issues.apache.org/jira/browse/MATH-904
>             Project: Commons Math
>          Issue Type: Bug
>    Affects Versions: 3.0
>            Reporter: Thomas Neidhart
>             Fix For: 3.1
>
>
> As reported by Jeff Hain:
> pow(double,double):
> Math.pow(-1.0,5.000000000000001E15) = -1.0
> FastMath.pow(-1.0,5.000000000000001E15) = 1.0
> ===> This is due to considering that power is an even
> integer if it is >= 2^52, while you need to test
> that it is >= 2^53 for it.
> ===> replace
> "if (y >= TWO_POWER_52 || y <= -TWO_POWER_52)"
> with
> "if (y >= 2*TWO_POWER_52 || y <= -2*TWO_POWER_52)"
> and that solves it.

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