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Suppose I have a code $C$ over $GF(2)$. I would like to count exactly the number of codewords of $C$ of weight $k$. Here $k$ should be thought of as small compared to the dimensions of the code.

What is the best algorithm for this? Even exponential algorithms could be useful.

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    $\begingroup$ Very closely related to this (unfortunately unanswered) question: cstheory.stackexchange.com/questions/19335/… $\endgroup$
    – usul
    Mar 22, 2014 at 14:35
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    $\begingroup$ (a reference there mentions that for general $k$ it is #P-complete, tel.archives-ouvertes.fr/docs/00/66/57/82/PDF/…) $\endgroup$
    – usul
    Mar 22, 2014 at 14:41
  • $\begingroup$ Note that it's NP-hard even to determine whether there are any codewords of weight $k$ [Vardy '97]. Therefore I suspect that essentially the best algorithm to solve OP's problem is the trivial $\Theta(n^k)$ one. Maybe some techniques from lattices would be useful to prove there's no subexponential algorithm under ETH. $\endgroup$
    – greg
    Mar 23, 2014 at 8:55

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