A k-combination with repetitions, or multisubset of size from a set is given by a sequence of not necessarily distinct elements of , where order is not taken into account: two sequences define the same multiset if one can be obtained from the other by permuting the terms. Associate an index to each element of and think of the elements of as types of objects, then we can let denote the number of elements of type in a multisubset. The number of multisubsets of size is then the number of nonnegative integer solutions of the Diophantine equation:
If has elements, the number of such -multisubsets is denoted by . This expression, multichoose , can also be given in terms of binomial coefficients:
.
In our case, the number of nonnegative solutions of the equation
is equal to
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