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 A083674 Triangular numbers whose sum of divisors is also a triangular number. 7
 1, 36, 45, 23220, 105111, 135460, 2492028, 5286126, 6604795, 14308575, 45025305, 50516326, 54742416, 99017628, 108125865, 152486916, 386767578, 1083567628, 1561818105, 3169234305, 5005551540, 5718242211, 6125307903, 6479715880 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 1, 5286126, 45025305, 54742416, ... are also hexagonal and their sum of divisors too. - Michel Marcus, Apr 20 2014 LINKS Donovan Johnson, Table of n, a(n) for n = 1..200 Shyam Sunder Gupta, Fascinating Triangular Numbers. Douglas E. Iannucci, Florian Luca, Triangular numbers whose sum of divisors is also triangular, Acta Arithmetica 129(2007), 23-40. EXAMPLE a(2)=36 because sum of divisors of 36 =1+2+3+4+6+9+12+18+36=91, which is also a triangular number. MATHEMATICA Select[Accumulate[Range[120000]], OddQ[Sqrt[8*DivisorSigma[1, #]+1]]&] (* Harvey P. Dale, Feb 25 2015 *) PROG (PARI) isok(n) = ispolygonal(n, 3) && ispolygonal(sigma(n), 3); \\ Michel Marcus, Apr 20 2014 CROSSREFS Cf. A008848 (similar with squares). Sequence in context: A264961 A188630 A167310 * A160063 A260927 A291713 Adjacent sequences:  A083671 A083672 A083673 * A083675 A083676 A083677 KEYWORD nonn AUTHOR Shyam Sunder Gupta, Jun 15 2003 STATUS approved

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Last modified October 29 12:00 EDT 2020. Contains 338066 sequences. (Running on oeis4.)