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Triangular Square NumberA triangular square number is a number which is both a triangular number and a perfect square. There is an infinity of triangular squares, given by the formula -
The problem of finding triangular square numbers reduces to Pell's equation in the following way. Every triangular number is of the form n(n − 1)/2. Therefore we seek integers n, m such that -
With a bit of algebra this becomes -
and then letting k = 2n − 1, we get the Diophantine equation -
which is an instance of Pell's equation. The kth triangular square Nk is equal to the sth perfect square and the tth triangular number, such that -
-
t is given by the formula - .
As k becomes larger, the ratio t/s approaches the square root of two: External references
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