Symmetry Rule for Binomial Coefficients/Proof 2
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Theorem
Let $n \in \Z_{>0}, k \in \Z$.
Then:
- $\dbinom n k = \dbinom n {n - k}$
Proof
From the definition of Cardinality of Set of Subsets, $\dbinom n k$ is the number of subsets of cardinality $k$ of a set with cardinality $n$.
Let $S$ be a set with cardinality $n$.
Let $\powerset S$ denote the power set of $S$.
Let $\AA_k \subseteq \powerset S$ be the set of elements of $\powerset S$ which have cardinality $k$.
For every $T \in \AA_k$ there exists its relative complement:
- $\relcomp S T \in \AA_{n - k}$
Define the mapping $f_k: \AA_k \to \AA_{n - k}$ as:
- $\forall T \in \AA_k: \map f T = \relcomp S T$
From Relative Complement Mapping on Powerset is Bijection it follows that $f_k$ is a bijection.
By definition of set equivalence, that means:
- $\card {A_k} = \card {A_{n - k} }$
That is, from the definition of Cardinality of Set of Subsets:
- $\dbinom n k = \dbinom n {n - k}$
$\blacksquare$
Sources
- 1966: Richard A. Dean: Elements of Abstract Algebra ... (previous) ... (next): $\S 0.6$: Theorem $8: \ 3$