Let us consider polynoms over $F_2$. Consider the linear SUBSPACE of polynoms divisible by $x^{16}+x^{12}+x^5 +1$ and of degree less or equal $N$ (e.g. 40).

Question: How many k-nomials belong to this subspace ?

By k-nomials I mean polynom containing only $k$ monomials, e.g. x^2+x - is 2-nomial.

Motivation and more general question

$g = x^{16}+x^{12}+x^5 +1$ is generating polynom for the CRC-16-CCITT error-correcting code. I am intersting about the Hamming weight distribution for the code-words, it is important characteristics of the code.

Question More generally we can take other "generating polynoms" and ask a similar questions, what is known about it ?

Examples

k =1,2 , answer = 0, rather obviously for all N.

k= 3 , answer = 0 , AFAIU (=as far as I understand)

k= 4 , answer N-15 , AFAIU