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This section cover combinatorics, and some binomial/multinomial facts.
The factorial of a number is or also as ,and is denoted by .
The number of ways of arranging ordered distinct objects is . This is also known as a permutation, and can be notated . We can see that this is true because there are objects which you can place in the first spot; when you've picked one there are objects to pick from for the second, and so on.
The number of ways of choosing objects from a set of objects without replacement (i.e. you can't pick an object twice) is , which is notated as either or . If you allow replacement, then it's notated and is given by . The reader should be able to deduce simple combinatorial arguments for these.
Binomials and Multinomials
The number of ways of ordering objects when of them are of one type, of them are of a second type, ... and of them of another type so that is
. The summation is taken over all sequences so that .
Balls and Urns
There are ways to divide objects in groups such that no group is empty and the objects are indistinguishable.