Combinatorial identity
Contents
[hide]Vandermonde's Identity
Vandermonde's Identity states that , which can be proven combinatorially by noting that any combination of objects from a group of objects must have some objects from group and the remaining from group .
Video Proof
https://www.youtube.com/watch?v=u1fktz9U9ig
Combinatorial Proof
Think of the right hand side as picking people from men and women. Think of the left hand side as picking men from the total men and picking women from the total women. The left hand side and right hand side are the same, thus Vandermonde's identity must be true.
~avn
Hockey-Stick Identity
For .
This identity is known as the hockey-stick identity because, on Pascal's triangle, when the addends represented in the summation and the sum itself is highlighted, a hockey-stick shape is revealed.
Proof
Inductive Proof
This identity can be proven by induction on .
Base Case Let .
.
Inductive Step Suppose, for some , . Then .
Algebraic Proof
It can also be proven algebraically with Pascal's Identity, . Note that
, which is equivalent to the desired result.
Combinatorial Proof 1
Imagine that we are distributing indistinguishable candies to distinguishable children. By a direct application of Balls and Urns, there are ways to do this. Alternatively, we can first give candies to the oldest child so that we are essentially giving candies to kids and again, with Balls and Urns, , which simplifies to the desired result.
Combinatorial Proof 2
We can form a committee of size from a group of people in ways. Now we hand out the numbers to of the people. We can divide this into disjoint cases. In general, in case , , person is on the committee and persons are not on the committee. This can be done in ways. Now we can sum the values of these disjoint cases, getting
Algebraic Proof 2
Apply the finite geometric series formula to : Then expand with the Binomial Theorem and simplify: Finally, equate coefficients of on both sides: Since for , , this simplifies to the hockey stick identity.
Another Identity
Hat Proof
We have different hats. We split them into two groups, each with k hats: then we choose hats from the first group and hats from the second group. This may be done in ways. Evidently, to generate all possible choices of hats from the hats, we must choose hats from the first and the remaining hats from the second ; the sum over all such is the number of ways of choosing hats from . Therefore , as desired.
Proof 2
This is a special case of Vandermonde's identity, in which we set .
Examples
- 1986 AIME Problem 11
- 2000 AIME II Problem 7
- 2013 AIME I Problem 6 (Solution 2)
- 2015 AIME I Problem 12
- 2020 AIME I Problem 7
- 2016 AMC 10A Problem 20
- 2021 AMC 12A Problem 15
- 1981 IMO Problem 2