Triangular Numbers Day - October 15

It's Triangular Numbers Day, the day we all joyously celebrate the triangular numbers! The triangular numbers are the number of figures needed to create an equilateral triangle. They are also the numbers representing the sum of each of the natural numbers:

0
0+1 = 1
0+1+2 = 3
0+1+2+3 = 6
0+1+2+3+4 = 10
etc.

This summation is probably the most useful characteristic of triangular numbers; after that they're probably more of a curiosity. A list of the start of the series can be found here.

October 15 was chosen as Triangular Numbers Day because 10/15 is the latest date that can be formed using consecutive triangular numbers.




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