Re: invertable matrix - result as integer matrix

[Thomas Weber]

On Sun, Jan 17, 2010 at 06:50:36AM +0100, Jaroslav Hajek wrote:
> On Sun, Jan 17, 2010 at 1:47 AM, orzech <address@hidden> wrote:
> >
> > Hi,
> > I would like to get an invertible matrix in Octave but as integers matrix,
> > so:
> >
> > x = [9,15;19,2];
> > inv(x)
> >
> > and the result is:
> >
> > [-0.0074906, 0.0561798; 0.0711610, -0.0337079]
> >
> > but I would like to get [22,17;25,21] anyone knows how to convert this
> > result? Many thanks.
> >
> 
> I don't understand:
> octave:7> [22,17;25,21] * [9,15;19,2]
> ans =
> 
>    521   364
>    624   417
> 
> this is clearly not remotely close to an inverse or its multiple.

I think orzech is calculating in the 26 residue class[1], i.e. Z/mZ with m=26.

mod(521,26) = 1;
mod(417,26) = 1;

mod(364,26) = 0;
mod(624,26) = 0;

[1] http://en.wikipedia.org/wiki/Modular_arithmetic

        Thomas