Octagonal Square Number

A number which is simultaneously octagonal and square. Let denote the th octagonal number and the th square number, then a number which is both octagonal and square satisfies the equation , or

(1)

Completing the square and rearranging gives

(2)

Therefore, defining

			(3)
			(4)

gives the Pell equation

(5)

The first few solutions are , (7, 4), (26, 15), (97, 56), (362, 209), (1351, 780), .... These give the solutions , (8/3, 4), (9, 15), (98/3, 56), (121, 209), ..., of which the integer solutions are (1, 1), (9, 15), (121, 209), (1681, 2911), ... (OEIS A046184 and A028230), corresponding to the octagonal square numbers 1, 225, 43681, 8473921, 1643897025, ... (OEIS A036428).

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