A basketball coach has 20 players available.
Out of them, he needs to choose five for the starting
lineup, and seven who would be sitting on the bench.
In how many ways can the coach choose these 5 plus 7 players?
It is certainly a huge number, but what exactly is it?
In this lecture, we will learn how to answer
questions of this kind.
More abstractly, we will develop methods for counting
the number of elements of a given set which is described
in some implicit way.
Now, why do we care?
The reason is that in many models, the calculation of
probabilities reduces to counting.
Counting the number of elements of various sets.
Suppose that we have a probability model in which the
sample space, Omega, is finite, and consists of n
equally likely elements.
So each element has probability 1/n.
Suppose now that we're interested in the probability
of a certain set, A, which has k elements.
Since each one of the elements of A has probability 1/n, and
since A has k distinct elements, then by the
additivity axiom, the probability of A is equal to k
times 1 over n.
Therefore to find the probability of A, all we have
to do is to count the number of elements of Omega and the
number of elements of A, and so determine the
numbers k and n.
Of course, if a set is described explicitly through a
list of its elements, then counting is trivial.
But when a set is given through some abstract
description, as in our basketball team example,
counting can be a challenge.
In this lecture, we will start with a powerful tool, the
basic counting principle, which allows us to break a
counting problem into a sequence of
simpler counting problems.
We will then count permutations, subsets,
combinations, and partitions.
We will see shortly what all of these terms mean.
In the process we will solve a number of example problems,
and we will also derive the formula for the binomial
probabilities, the probabilities that describe
the number of heads in a sequence of
independent coin tosses.
So, let us get started.