by Jonathan Kujawa
Last month at 3QD we discovered that Pascal's Triangle contains all sorts of surprises. Like most things in mathematics, there is no end to the things you can uncover if you keep digging and have a curious mind. If we revisit the Triangle with our eye open for curiosities we notice that the sequences of numbers which run parallel to the side of the triangle look a bit interesting [1]:
Okay, the first line is just a sequence of 1s and is pretty darn boring. The second sequence is the counting numbers and is only slightly better than the 1s since anyone over the age of five who knows the addition rule for the Triangle can see why they are there.
But the third row! They seem to follow some sort of pattern, but it's not quite so obvious what it might be. We already have the hint that they're called the Triangular Numbers. If you were a boring person devoid of curiosity, you could go on with your life never knowing what's going on. But you're not and you know the Triangle rewards the curious. The following image explains why they're called the Triangular Numbers:
Once you notice that the pattern, it's not too hard to see that the nth number in this sequence is the number of balls needed to make a triangle which has n balls along one side (to go from one triangle to the next you just add another row to one side and counting those additional balls amounts to the addition rule of Pascal's Triangle). It's also not too hard to see [2] that the nth Triangular Number is given by the formula n(n+1)/2.
Let's agree that the 0th triangular number is 0. Not only is it reasonable to say the triangle with no balls on each side is made from 0 balls, we'll see it also turns out to be a convenient convention.