A number which is simultaneously a heptagonal number and triangular number . Such numbers exist when
(1)
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Completing the square and rearranging gives
(2)
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Substituting and gives the Pell-like quadratic Diophantine equation
(3)
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which has basic solutions , (7, 3), and (18, 8). Additional solutions can be obtained from the unit Pell equation, and correspond to integer solutions when , (5, 10), (221, 493), (1513, 3382), ... (OEIS A046193 and A039835), corresponding to the heptagonal triangular numbers 1, 55, 121771, 5720653, 12625478965, ... (OEIS A046194).