[GiNaC-list] using modulo and expanding

[C.Dams at science.ru.nl]

Dear Kostas,

Citeren Kostas Patsakis <kpatsak at gmail.com>:

> is there a way to use modulo over an expression?
> for example:
> 3*k1^2+4*k1*k2+1231231*k1 mod 2=
> k1^2+k1
> 
> all I really want is to work with polynomials over Z_2
> so with subs I can do subs(a,$1^2=$1)

This does not sound like a good idea. From the above I gather that you do not 
want that you substitution gives k1^2 -> k1 but with the substitution given 
here you will get this result.

> but some times it's not enough cause I might run into something with power
> more than 2 would subs(a,$1^$2=$1) be enough???
> I'm not using ginsh but c++
> any help???

Try a map function. Something like (not tested, so probably some things are 
wrong):

struct Do_mod_2 : public map_function
{  ex operator()(const ex&x)
   {  if(is_a<numeric>(x))
         return mod(ex_to<numeric>(x),2);
      return x.map(*this);
   }
};

Do_mod_2 do_mod_2;

You can now use do_mod_2(3*k1^2+4*k1*k2+1231231*k1);

A happy new year!
Chris