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How many unique ordered lists of $N$ integers - $(q_1, ..., q_N)$ - can I form if $q_i \in [0, M]$, and we have the restriction that $\sum_{i=0}^{N} q_i \geq k$?

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What is $Q$? Perhaps that should be removed, as well as "possible unique". –  leonbloy Dec 24 '12 at 13:41
    
@leonbloy Q is an ordered set of integers, for example, when N=3, we have: {q1, q2, q3}. –  TLi Dec 24 '12 at 13:42
    
@leonbloy I have made the requested changes. Hopefully this should be better? –  TLi Dec 24 '12 at 13:45
    
Ok. Two more clarifications: the order must be strict? (no equal integers? in this case the problem could be stated in combinatoric terms). Second: the notation $q_i \in [0, M]$ is equivalent to $q_i= 0, 1\cdots M$? –  leonbloy Dec 24 '12 at 13:48
    
@leonbloy Integers may be equal, however {10, 5} must be counted as distinct from {5, 10}, for example. Yes, regarding the second point. –  TLi Dec 24 '12 at 13:51

1 Answer 1

The number of such lists is the sum of the coefficients of $x^r$ for $r \ge k$ in the expansion of $$(1+x+...+x^M)^N.$$ There may be a clever way to extract the given sum of coefficients, but I don't see it. Anyway if you have a specific case to calculate, and can use maple or other CAS, the count can be found; maple for example has a function to extract a coeffient of a power of $x$ from an expanded polynomial, and one could code up a function to sum those for $r\ge k$.

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