And of course ancient mathematicians knew it already at least around 2000 years ago: https://en.wikipedia.org/wiki/Squared_triangular_number https://en.wikiped

And of course ancient mathematicians knew it already at least around 2000 years ago: https://en.wikipedia.org/wiki/Squared_triangular_number https://en.wikipedia.org/wiki/Squared_triangular_number "Pengelley (2002) finds references to the identity not only in the works of Nicomachus in what is now Jordan in the first century CE" Now it is Jordan, but at the time on Nicomachus there were almost 300 years of Greek and 100 years of Roman rule of that area. https://en.wikipedia.org/wiki/Nicomachus https://en.wikipedia.org/wiki/Nicomachus