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Let $f(0)=f(1)=0$ and \[f(n+2)=4^{n+2}f(n+1)-16^{n+1}f(n)+n.2^{n^2},\; \; \; \; n\epsilon \mathbb{N}_0\]
Write $f(n)$ in terms of $n$.

u modified nothing.just a part of the problem.this can be done defining another sequence in terms of f and then telescoping twice.

zadid xcalibured wrote:u modified nothing.just a part of the problem.this can be done defining another sequence in terms of f and then telescoping twice.

Well, I didn't solve it in that way.I solved it by starting to find residues from $2$ to $1991$.That's why I said this "modified".

will u post ur solution?