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%I #17 Jul 13 2013 12:03:48
%S 0,1,3,6,10,12,15,19,21,24,28,30,33,37,42,46,51,55,60,64,69,73,78,82,
%T 87,91,96,100,102,105,109,111,114,118,120,123,127,132,136,141,145,150,
%U 154,159,163,168,172,177,181,186,190,195,201,204,208,210,213,217
%N Numbers whose digital sum is a triangular number.
%C Every term with some permutations can become another term of this sequence.
%C The subsequence of primes begins: 3, 19, 37, 73, 91, 127...
%C The subsequence of triangular numbers begins: 1, 3, 6, 10, 15, 21, 28, 55...
%H Reinhard Zumkeller, <a href="/A187744/b187744.txt">Table of n, a(n) for n = 1..10000</a>
%F If decimal expansion of n is x1 x2 ... xk then x1 + x2 + ... xk = T.
%F A010054(A007953(a(n))) = 1. - _Reinhard Zumkeller_, Jan 03 2013
%t TriangularQ[n_] := IntegerQ[Sqrt[1 + 8 n]]; Select[Range[0, 300], TriangularQ[Total[IntegerDigits[#]]] &] (* _T. D. Noe_, Jan 03 2013 *)
%o (Haskell)
%o a187744 n = a187744_list !! (n-1)
%o a187744_list = filter ((== 1) . a010054 . a007953) [0..]
%o -- _Reinhard Zumkeller_, Jan 03 2013
%Y Cf. A000217, A007960, A000027.
%K nonn,base
%O 1,3
%A _Dario Piazzalunga_, Jan 03 2013
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