- 1.
- Multiplication Rule:

If k successive choices are to be made with exactly*n*_{j}choices at each stage, then the total number of successive choices is

- 2.
- Number of orderings:

An ordering (or arrangement or permutation of k out of n) is a sequence of length k out of n choices with no duplications. The number of such arrangements is:

. - 3.
- Permutations:
If the total number of possible choices is the same
as the number of elements we want to pick,
this is called a permutation, there are :

permutations of n elements.We have a special way of calling this number: ``factorial n'' =

*n*! This number gets big very fast: 3!=6, 5!= 120, 10!=3628800, 20!= 2432902008176640000,

50!=30414093201713378043612608166064768844377641568960512000000000000. - 4.
- Combinations:

The number of ways of choosing k out of n objects is:

Property 1:

Property 2:

- Birthday Problem 10/6
- Approximating
*n*! 10/7 - A note on doing symbolic computations
- More about k-subsets 10/9
- The matching Problem 10/9
- Multinomial Coefficients and applications 10/12