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 A187744 Numbers whose digital sum is a triangular number. 1
 0, 1, 3, 6, 10, 12, 15, 19, 21, 24, 28, 30, 33, 37, 42, 46, 51, 55, 60, 64, 69, 73, 78, 82, 87, 91, 96, 100, 102, 105, 109, 111, 114, 118, 120, 123, 127, 132, 136, 141, 145, 150, 154, 159, 163, 168, 172, 177, 181, 186, 190, 195, 201, 204, 208, 210, 213, 217 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Every term with some permutations can become another term of this sequence. The subsequence of primes begins: 3, 19, 37, 73, 91, 127... The subsequence of triangular numbers begins: 1, 3, 6, 10, 15, 21, 28, 55... LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 FORMULA If decimal expansion of n is x1 x2 ... xk then x1 + x2 + ... xk = T. A010054(A007953(a(n))) = 1. - Reinhard Zumkeller, Jan 03 2013 MATHEMATICA TriangularQ[n_] := IntegerQ[Sqrt[1 + 8 n]]; Select[Range[0, 300], TriangularQ[Total[IntegerDigits[#]]] &] (* T. D. Noe, Jan 03 2013 *) PROG (Haskell) a187744 n = a187744_list !! (n-1) a187744_list = filter ((== 1) . a010054 . a007953) [0..] -- Reinhard Zumkeller, Jan 03 2013 CROSSREFS Cf. A000217, A007960, A000027. Sequence in context: A316325 A114981 A319452 * A007960 A280243 A032732 Adjacent sequences:  A187741 A187742 A187743 * A187745 A187746 A187747 KEYWORD nonn,base AUTHOR Dario Piazzalunga, Jan 03 2013 STATUS approved

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Last modified December 16 09:06 EST 2019. Contains 330020 sequences. (Running on oeis4.)