roots of polynomials

Hi

I'm trying to compute roots of polynomial given by recurence
relation

My polynomial :
P(n):=if n=0 then 0 else P(n-1)2+c;

I'm using allroots
see: http://fraktal.republika.pl/mset_centers.html
but for n>6 roots seems to be wrong to me.

Can it be done better ?

Adam Majewski