Many counting problems are solved by establishing a bijection between the set to be counted and some easy-to-count set. This kind of proofs are usually called (non-rigorously) combinatorial proofs.
The number of k-compositions of n is equal to the number of solutions to in positive integers.
count the number of solutions to in nonnegative integers We call such a solution a weak k-composition of n.
Formally, a multiset M on a set S is a function . For any element , the integer is the number of repetitions of x in M, called the multiplicity of x. The sum of multiplicities is called the cardinality of M and is denoted as | M | .
we have already evaluated the number . If , let zi = m(xi), then is the number of solutions to in nonnegative integers, which is the number of weak n-compositions of k, which we have seen is .
We can think of it as that n labeled balls are assigned to m labeled bins, and is the number of assignments such that the i-th bin has ai balls in it.