Permutations/combinations problem

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I have a problem about combinations and permutations I am trying to solve. Say we have an n-dimensional vector. Each element of the vector can contain any one of [tex]\lambda=3[/tex] values (-1, 0 or +1). Then the number of possible vectors is simply:


If we place the additional restriction that the vector must contain exactly [tex]k[/tex] non-zeros, then it becomes:


If we change the restriction so that it must contain at most [tex]k[/tex] non-zeros and at least 1 non-zero, then it becomes:


Are my equations correct? Is there a more compact way of expressing this last equation, to get rid of the summation?


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The formulas are correct.

If we want to get rid of the sum, we needed ##\binom n {k'} =\binom k {k'} \cdot r(n,k)## but the correction ##r## does also depend on ##k'##. So I guess there is no way to get rid of the sum.

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