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 A213517 Numbers n such that the triangular number n*(n+1)/2 has only 1 or 2 different digits in base 10. 4
 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 18, 24, 34, 36, 44, 58, 66, 77, 100, 101, 105, 109, 114, 132, 141, 363, 666, 714, 816, 1000, 1095, 1287, 1332, 1541, 3363, 6666, 10000, 10114, 13332, 66666, 100000, 133332, 666666, 1000000, 1333332, 6666666, 10000000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The list of triangular numbers containing only one digit (A045914) is finite. This list is infinite because numbers like 133332, 666666, and 1000000 occur an infinite number of times. Are there more terms not of this form? See A213516 for corresponding triangular numbers. A118668(a(n)) <= 2. - Reinhard Zumkeller, Jul 11 2015 LINKS MATHEMATICA t = {}; Do[tri = n*(n+1)/2; If[Length[Union[IntegerDigits[tri]]] <= 2, AppendTo[t, n]], {n, 0, 10^5}]; t PROG (Haskell) a213517 n = a213517_list !! (n-1) a213517_list = filter ((<= 2) . a118668) [0..] -- Reinhard Zumkeller, Jul 11 2015 CROSSREFS Cf. A045914, A213516. Cf. A118668. Sequence in context: A154125 A106039 A151767 * A173899 A108897 A023767 Adjacent sequences:  A213514 A213515 A213516 * A213518 A213519 A213520 KEYWORD nonn,base AUTHOR T. D. Noe, Jun 21 2012 STATUS approved

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Last modified May 21 17:17 EDT 2019. Contains 323444 sequences. (Running on oeis4.)