nmod_poly_extrapolate_geometric, nmod_poly_extrapolate_geometric_precomp

Precisely, for any invertible $c$ and $k \ge 0$ (which the functions need not know), if ival[i] $= f(c \cdot r^{2(k+i)})$ for $0 \le i < \mathit{ilen}$, then oval[j] $= f(c \cdot r^{2(\ell + j)})$ for $0 \le j < \mathit{olen}$ with $\ell = k + \mathit{offset}$. This uses one middle product. In the forward case none of $r^2, \ldots, r^{2(\mathit{offset} + \mathit{olen} - 1)}$ may be 1, and G->len must be at least $\mathit{offset} + \mathit{olen}$; in the backward case the same holds for $\mathit{ilen} - \mathit{offset}$. G must be prepared for extrapolation; the function without _precomp builds a temporary G. For $\mathit{ilen} \le 1$ the output is constant.