TheoremBase

Counting Tuples with a Given Sum

There are C(n+k,k) tuples of k+1 natural numbers with sum n, and as many tuples of k+1 positive integers with sum n+k+1.

Statement

In the setting of Finite Combinatorics: Notation, let n,k∈N0n,k\in\mathbb{N}_{0}, and let tuples and their components be as in Tuples in a Set: the Set of n-Tuples and Their Components (i) and Tuples in a Set: the Set of n-Tuples and Their Components (iii).

Nonnegative entries.

#{x∈N0k+1:x1+⋯+xk+1=n}=(n+kk).\#\{x\in\mathbb{N}_{0}^{k+1}:x_{1}+\dots+x_{k+1}=n\}=\binom{n+k}{k}.

Positive entries.

#{x∈Nk+1:x1+⋯+xk+1=n+k+1}=(n+kk).\#\{x\in\mathbb{N}^{k+1}:x_{1}+\dots+x_{k+1}=n+k+1\}=\binom{n+k}{k}.

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