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Karim Belabas on Thu, 12 Jan 2023 13:14:40 +0100
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Re: probable bug of "ispower" function in GP: FALSE, apologies!
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- To: luis.gallardo@math.cnrs.fr
- Subject: Re: probable bug of "ispower" function in GP: FALSE, apologies!
- From: Karim Belabas <Karim.Belabas@math.u-bordeaux.fr>
- Date: Thu, 12 Jan 2023 13:13:22 +0100
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* Vincent Lefevre [2023-01-12 12:40]:
> On 2023-01-12 12:31:09 +0100, luis.gallardo@math.cnrs.fr wrote:
> > I misunderstood the definition of "ispower"!
> >
> > I wanted a function that identify directly if some integer is a power of 2
>
> I suppose that you can use something like
>
> ispower(64,,&n)
>
> and check that n is 2. But I suppose that for large numbers, this may
> be inefficient because ispower() will not just check for powers of 2.
Use hammingweight(N) == 1.
Or v = valuation(N,2); N >> v == 1 if you need the exact power. (Will be
a little slower.)
Cheers,
K.B.
--
Pr Karim Belabas, U. Bordeaux, Vice-président en charge du Numérique
Institut de Mathématiques de Bordeaux UMR 5251 - (+33) 05 40 00 29 77
http://www.math.u-bordeaux.fr/~kbelabas/
`