2 Tools for Dealing With Partitions 2.1 Ferrers Diagrams
A Ferrers diagram is a way of visualizing partitions with dots. Each row represents one addend in the partition. The number of dots in a row represents the value of that total addend. For example, the partition of 10 into 5+3+1+1 is shown below.
5••••• 3•••
1•
1•
Example 1. Show that the number of partitions of an integer n into parts the largest of which is r is equal to the number of partitions of n into exactly r parts.
Solution. We are trying to find a way to relate two different types of partitions of n both in terms of r. Perhaps a Ferrers diagram could lead us in the right direction. So let us try some examples.
Suppose n = 10 and r = 3. Then one partition of n in which r is the largest part is 3+3+2+1+1. In a Ferrers diagram this looks like:
3••• 3••• 2•• 1•
1•
The numbers on the left side of the diagram are found by counting up the dots in the row next
to it. However, consider what would happen if we put numbers at the top and counted the dots in the columns. This would look like: