Two And Two Make Four. By Simon Donaldson
By Simon Donaldson If we have four objects—say A, B, C, D—there are just three ways of dividing them into pairs: (AB)(CD) (AC)(BD) (AD)(BC). The salient property is that 3 is less than 4. This simple fact expresses something special about the number 4. For example if we take 6 objects there are 10 ways to divide … Read more