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From: "Patricia Shanahan" <patricia.shanahan@1:261/38.remove-dpk-this>
Subject: Re: What is the command to do a power of a value
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Date: Tue, 31 Jul 2012 18:02:33 GMT
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  To: Andreas Leitgeb
From: "Patricia Shanahan" <patricia.shanahan@1:261/38.remove-rf4-this>

  To: Andreas Leitgeb
From: Patricia Shanahan <pats@acm.org>

On 7/30/2012 6:22 AM, Andreas Leitgeb wrote:
> On Sun, 29 Jul 2012 21:28:11 -0700 (PDT), xvictoryeohx@gmail.com wrote:
>> C=L(1+i/100)power of n
>
>   x ^ n  =  exp ( log(x) * n )    |  x = (1 + i/100)
>          = exp ( log( 1 + i/100 ) * n)
>          = exp ( log1p ( i/100 ) * n)
>
> If you're doing more calculations with same interest-rate but
> different periods, then you may want to calculate
>    double logBase = Math.log1p( i / 100 );
> once, and use that for the individual calculations:
>    C = L * Math.exp( logBase * n )
>
> The gist of this response is, that for the kind of base (1+i/100),
> you better separate the pow operation out into log and exp, and
> actually use log1p on (i/100) instead of log on (1+i/100) for
> efficiency's and precision's sake.
>
> For "Math.log1p" see:
> http://docs.oracle.com/javase/6/docs/api/java/lang/Math.html#log1p%28double%2
9
>

I am curious about why you expect this to be more precise than: "The computed 
result must be within 1 ulp of the exact result. Results must be 
semi-monotonic." (From the pow description). Or do you know of cases where 
Math.pow gets an over-large rounding error?

Note that I am not disagreeing with your method for calculating a power of a 
number slightly greater than 1, just questioning whether doing it explicitly 
gets more precise answers than using Math.pow.

Patricia

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