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From "Thomas Neidhart (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MATH-904) FastMath.pow deviates from Math.pow for negative, finite base values with an exponent 2^52 < y < 2^53
Date Tue, 20 Nov 2012 20:52:58 GMT
Thomas Neidhart created MATH-904:
------------------------------------

             Summary: 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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