2-adic roots (Re: bdiv vs redc)

Niels Möller nisse at lysator.liu.se
Fri Jul 20 08:26:07 CEST 2012

Replying to myself again...

> 2. An odd number has a square root (or in fact two) if and only if it's
>    = 1 (mod 4).

It seems I have some difficulty getting this right. There are four
square roots of unity, not just two. Namely,

  2^{k-1} - 1
  2^{k-1} + 1
  2^k - 1

And the numbers which are squares are 1 (mod 8).


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