For integers $n\ge r \ge 0$ we define $$ {n\choose r} = \frac{n!}{(n-r)!r!} $$ and call such numbers binomial coefficients.

Properties.

1. $n\choose r$ is an integer (proof.).
2. ${n\choose r}={n\choose n-r}$ .
3. ${n\choose r-1}+{n\choose r}={n+1\choose r}$ (Pascal's rule).
4. ${n\choose 0}=1={n\choose n}$ for all $n$ .
5. ${n\choose 0}+{n\choose 1}+{n\choose 2}+\cdots+{n\choose n}=2^n$ .
6. ${n\choose 0}-{n\choose 1}+{n\choose 2}-\cdots+(-1)^n{n\choose n}=0$ for $n>0$ .
7. $\sum_{t=1}^n {t\choose k }={n+1\choose k+1}$ .

Properties 5 and 6 are the binomial theorem applied to $(1+1)^n$ and $(1-1)^n$ , respectively, although they also have purely combinatorial meaning.

Motivation

• $n\choose r$ constitute the coefficients when expanding the binomial $(x+y)^n$ - hence the name binomial coefficients. See Binomial Theorem.
• $n\choose r$ is the number of ways to choose $r$ objects from a set with $n$ elements.
• On the context of computer science, it also helps to see ${n\choose r}$ as the number of strings consisting of ones and zeros with $r$ ones and $n-r$ zeros. This equivalency comes from the fact that if $S$ be a finite set with $n$ elements, ${n\choose r}$ is the number of distinct subsets of $S$ with $r$ elements. For each subset $T$ of $S$ , consider the function $$ X_T\colon S \rightarrow \{0,1\} $$ where $X_T(x) = 1$ whenever $x \in T$ and $0$ otherwise (so $X_T$ is the characteristic function for $T$ ). For each $T \in \mathcal{P}(S)$ , $X_T$ can be used to produce a unique bit string of length $n$ with exactly $r$ ones.

Notes

Bibliography

1

N. Higham, Handbook of writing for the mathematical sciences, Society for Industrial and Applied Mathematics, 1998.