## What is a combinatorial identity?

A combinatorial identity is proven by counting the number of elements of some carefully chosen set in two different ways to obtain the different expressions in the identity. Since those expressions count the same objects, they must be equal to each other and thus the identity is established.

## How do you write a combinatorial identity?

In general, to give a combinatorial proof for a binomial identity, say A=B you do the following:

- Find a counting problem you will be able to answer in two ways.
- Explain why one answer to the counting problem is A. A .
- Explain why the other answer to the counting problem is B. B .

**What is a combinatorial explanation?**

Definition: A combinatorial interpretation of a numerical quantity is a set of combinatorial objects that is counted by the quantity. You find a set of objects that can be interpreted as a combinatorial interpretation of both the left hand side (LHS) and the right hand side (RHS) of the equation.

**Are combinatorial proofs rigorous?**

Combinatorics certainly can be rigourous but is not usually presented that way because doing it that way is: longer (obviously) less clear because the rigour can obscure the key ideas. boring because once you know intuitively that something works you lose interest in a rigourous argument.

### How do you do combinatorial proof?

In general, to give a combinatorial proof for a binomial identity, say A=B you do the following:

- Find a counting problem you will be able to answer in two ways.
- Explain why one answer to the counting problem is A.
- Explain why the other answer to the counting problem is B.

### What does 5 choose 3 mean?

5C3 or 5 choose 3 refers to how many combinations are possible from 5 items, taken 3 at a time. What is a combination? Just the number of ways you can choose items from a list.

**What are some good resources for learning about combinatorial identities?**

In general, Gould’s work is a great resource for this sort of thing; he has spent much of his career collecting and proving combinatorial identities. Added: Another useful reference is John Riordan’s Combinatorial Identities.

**What are some good resources for learning binomial coefficient identities?**

Added: Another useful reference is John Riordan’s Combinatorial Identities. It’s hard to pick one of its 250 pages at random and notfind at least one binomial coefficient identity there. Unfortunately, the identities are not always organized in a way that makes it easy to find what you are looking for. Still it’s a good resource.

## What is a binomial identity?

Each of these is an example of a binomial identity : an identity (i.e., equation) involving binomial coefficients. Our goal is to establish these identities.

## What is the best way to present important formulas in combinatorics?

The question collects important formulas representing major progress in combinatorics. 2) Present the formula explicitly (not just by name or by a link or reference), and briefly explain the formula and its importance, again not just link or reference. (But then you may add links and references.)