# Re: [GAP Forum] Sqrt for the cyclotomic numbers

```On Tue, Jan 20, 2015 at 07:31:56AM +0000, Palcoux Sebastien wrote:
> Dear Alexander and Forum,
> If the cyclotomic number is the square of a cyclotomic number, is there an
> easy way to find it?
> The number I need are the eigenvalues of the matrix of the unitarized inner
> product of an irreducible representation of a finite group (see the comment
> of Paul Garett here: http://math.stackexchange.com/q/1107941/84284). This
> matrix is positive, I guess its eigenvalues are always cyclotomic (true for
> the examples I've looked, but I don't know in general), and I hope they are
> square of cyclotomic. Thanks to these square roots I can compute the unitary
> matrices for the irreducible representation.```
```
You don't need to take square roots. If H is the Hermitian positive definite
form
you obtained by the averaging (or in some other way) then H=LDL*, for
L a lower-triangular matrix with 1s on the main diagonal, and D is a diagonal
matrix.
L and D can be computed without taking square roots (and so they will stay
cyclotomic).
Then conjugating by L gives you the unitary form.

HTH,
Dmitrii

> Remark: a function on GAP computing the unitary irreducible representations
> seems very natural, so if there is not such a function, this should means
> that there are problems for computing them in general with GAP, isn't it?
> Best regards,Sebastien Palcoux
>
>      Le Mardi 20 janvier 2015 3h13, Alexander Hulpke <hul...@fastmail.fm> a
> écrit :
>
>
>  Dear Forum,
>
> > On Jan 19, 2015, at 1/19/15 2:18, Palcoux Sebastien
> > <sebastienpalc...@yahoo.fr> wrote:
> >
> > Hi,
> > Is it possible to extend the function Sqrt on the cyclotomic numbers?
>
> How would you represent this root? In general the square root of a cylotomic
> is not cyclotomic again. (You could form a formal AlgebraicExtension, but
> then you lose the irrational cyclotomics for operations.)
>
> Regards,
>
>   Alexander Hulpke

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