Method of distinguished element

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"Distinguished element" redirects here. For sets with pre-defined distinguished elements, see Pointed set.

In enumerative combinatorial mathematics, identities are sometimes established by arguments that rely on singling out one "distinguished element" of a set.

[edit] Examples

  • The binomial coefficient {n \choose k} is the number of size-k subsets of a size-n set. A basic identity, one of whose consequences is that these are precisely the numbers appearing in Pascal's triangle, states that:
{n \choose k-1}+{n \choose k}={n+1 \choose k}.
Proof: In a size-(n + 1) set, choose one distinguished element. The set of all size-k subsets contains: (1) all size-k subsets that do contain the distinguished element, and (2) all size-k subsets that do not contain the distinguished element. If a size-k subset of a size-(n + 1) set does contain the distinguished element, then its other k − 1 elements are chosen from among the other n elements of our size-(n + 1) set. The number of ways to choose those is therefore {n \choose k-1}. If a size-k subset does not contain the distinguished element, then all of its k members are chosen from among the other n "non-distinguished" elements. The number of ways to choose those is therefore {n \choose k}.
  • The number of subsets of any size-n set is 2n.
Proof: We use mathematical induction. The basis for induction is the truth of this proposition in case n = 0. The empty set has 0 members and 1 subset, and 20 = 1. The induction hypothesis is the proposition in case n; we use it to prove case n + 1. In a size-(n + 1) set, choose a distinguished element. Each subset either contains the distinguished element or does not. If a subset contains the distinguished element, then its remaining elements are chosen from among the other n elements. By the induction hypothesis, the number of ways to do that is 2n. If a subset does not contain the distinguished element, then it is a subset of the set of all non-distinguished elements. By the induction hypothesis, the number of such subsets is 2n. Finally, the whole list of subsets of our size-(n + 1) set contains 2n + 2n = 2n+1 elements.
  • Let Bn be the nth Bell number, i.e., the number of partitions of a set of n members. Let Cn be the number of "parts" (or "blocks", as combinatorialists often call them) among all partitions of that set. For example, the partitions of the size-3 set {a, b, c} may be written thus:
\begin{matrix}abc \\ a/bc \\ b/ac \\ c/ab \\ a/b/c \end{matrix}
We see 5 partitions, containing 10 blocks, so B3 = 5 and C3 = 10. An identity states:
B_n+C_n=B_{n+1}.\,
Proof: In a size-(n + 1) set, choose a distinguished element. In each partition of our size-(n + 1) set, either the distinguished element is a "singleton", i.e., the set containing only the distinguished element is one of the blocks, or the distinguished element belongs to a larger block. If the distinguished element is a singleton, then deletion of the distinguished element leaves a partition of the set containing the n non-distinguished elements. There are Bn ways to do that. If the distinguished element belongs to a larger block, then its deletion leaves a block in a partition of the set containing the n non-distinguished elements. There are Cn such blocks.