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 A117523 Triangular numbers for which the sum of the digits is an octagonal number. 1
 0, 1, 10, 1596, 2775, 3486, 3828, 4278, 4656, 5565, 6555, 7626, 8256, 9453, 14196, 15753, 16653, 17391, 18336, 21945, 22791, 23871, 24753, 28920, 32385, 34716, 37128, 38226, 39621, 40755, 42195, 43365, 44850, 46056, 51681, 54615, 56280 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS Michael De Vlieger, Table of n, a(n) for n = 1..3618 EXAMPLE 1596 is in the sequence because (1) it is a triangular number and (2) the sum of its digits 1+5+9+6=21 is a octagonal number. MATHEMATICA Join[{0}, Select[Accumulate[Range[400]], IntegerQ[(1+Sqrt[1+3*Total[ IntegerDigits[ #]]])/3]&]] (* Harvey P. Dale, May 06 2019 *) PROG (PARI) isok(n) = ispolygonal(n, 3) && ispolygonal(sumdigits(n), 8); \\ Michel Marcus, Feb 26 2014 CROSSREFS Cf. A000217, A000567. Sequence in context: A286397 A160236 A204466 * A203696 A203530 A232594 Adjacent sequences:  A117520 A117521 A117522 * A117524 A117525 A117526 KEYWORD base,nonn AUTHOR Luc Stevens (lms022(AT)yahoo.com), Apr 30 2006 STATUS approved

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Last modified August 3 10:39 EDT 2021. Contains 346435 sequences. (Running on oeis4.)