Poly invmodxn(const Poly& a,int n)
{ // Newton's method to find 1/a mod x^n
int i,k;
Poly b;
k=0; while ((1<<k)<n) k++;
b.addterm((ZZn)1/a.coeff(0),0); // important that a0 != 0
for (i=1;i<=k;i++) b=modxn (2*b-a*b*b,1<<i);
b=modxn(b,n);
return b;
}
BOOL isone(const Poly& p)
{
if (degree(p)==0 && p.coeff(0)==1) return TRUE;
else return FALSE;
}
