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Square triangular number
Quote:
In mathematics, a square triangular number (or triangular square number) is a number which is both a triangular number and a perfect square..In 1778 Leonhard Euler determined the explicit formula..The problem of finding square triangular numbers reduces to Pell's equation..Euler was featured on the sixth series of the Swiss 10franc banknote and on numerous Swiss, German, and Russian postage stamps.. He is also commemorated by the Lutheran Church on their Calendar of Saints on 24 May – he was a devout Christian (and believer in biblical inerrancy) who wrote apologetics and argued forcefully against the prominent atheists of his time..The asteroid 2002 Euler was named after him...

http://en.wikipedia.org/wiki/Square_triangular_number
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