Applying one of his favorite materials – checkers – Simon showed me the tricks behind polygonal numbers. The numbers written in pen (above) correspond to the actual triangle number (red rod) and the row number (blue rod).Square numbersPentagonal numbersAnd the next pentagonal number(Centered) Hexagonal numbersFragment of the next (centered) hexagonal numberThe following morning I saw that Simon came up with these general formulae to construct square, pentagonal and hexagonal numbers using triangle numbers. The n stands for the index of the polygonal number. Later Simon told me that he had made a mistake in his formula for the hexagonal numbers: it should not be the ceiling function of (n-1)/2, but simply n-1, he said.
I asked Simon to show me how he’d come up with the formulae:
Here is a square number constructed of two triangle numbers (the 5th and the 4th, so the nth and the n-1st)The working out of the same construction. In the axample above n equals 5, so the 5th square number is indeed 25.The nth pentagonal number constructed using three triangle numbers: the nth triangle number, and two, n-1st triangle numbers.The working out of the pentagonal number formulaThe nth hexagonal numberThe formula for calculating the nth hexagonal number from six n-1st triangle numbers plus 1. (Simon later corrected the (n+1) into (n-1)).
Geeks out of the box 