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 A185129 Smaller member of a pair of triangular numbers whose sum and difference are triangular. 7
 15, 105, 378, 780, 2145, 1485, 5460, 7875, 29403, 21945, 70125, 105570, 61425, 37950, 255255, 306153, 61425, 667590, 749700, 522753, 1016025, 353220, 176715, 1471470, 1445850, 1747515, 246753, 794430, 749700, 514605, 3499335, 2953665, 5073705, 635628, 8382465 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES Albert H. Beiler, Recreations in the theory of numbers, New York, Dover, (2nd ed.) 1966, p. 197, nr. 8. LINKS EXAMPLE a(2) = 105, since the pair of triangular numbers 171 = 18*(18+1)/2 and 105 = 14*(14+1)/2 produce the sum 276 = 23*(23+1)/2 and the difference 66 = 11*(11+1)/2 which are both triangular numbers. PROG (PARI) lista(n) = {v = vector(nn, n, n*(n+1)/2); for (n=2, nn, for (k=1, n-1, if (ispolygonal(v[n]+v[k], 3) && ispolygonal(v[n]-v[k], 3), print1(v[k], ", ")); ); ); } \\ Michel Marcus, Jan 08 2015 CROSSREFS Cf. A000217, A185128, A185223, A185233, A185243, A185253, A185257, A185258. Sequence in context: A160892 A061550 A174385 * A090454 A240440 A256287 Adjacent sequences:  A185126 A185127 A185128 * A185130 A185131 A185132 KEYWORD nonn AUTHOR Martin Renner, Jan 20 2012 STATUS approved

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Last modified May 31 10:42 EDT 2020. Contains 334748 sequences. (Running on oeis4.)