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 A118711 Integers k such that the k-th triangular number t_k has all its base-12 digits contained in {1,5,7,11}. 0
 1, 13, 61, 82, 898, 2962, 2989, 9133, 20077, 20653, 28669, 29266, 35581, 35842, 37501, 99133, 236674, 286717, 424621, 424957, 821698, 941650, 1704301, 1722370, 2978413, 3328258, 4494466, 10022317, 40392829, 49870141, 50668882, 53933053 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS In base 12 all primes greater than 3 end in the digits 1, 5, 7, E, where X is 10 and E is 11. They are the digits that satisfy GCD(d,12)=1. The sequence in base 12 is: 1, 11, 51, 6X, 62X, 186X, 1891, 5351, E751, EE51, 14711, 14E2X, 18711, 188XX, 19851, 49451, E4E6X, 119E11, 185891, 185E11, 33762X, 394E2X, 6X2351, 6E08XX, EE7751, 11460XX, 1608E6X, 3433E51, 1163E591, 14850051, 14E7632X, 1608E311, 18331451, 1870E191, 1974E311, ..., . Note that all elements end in 1 or X. The corresponding triangular numbers after the first end in the digits 17 or 77, but not respectively. LINKS FORMULA k is a term if the k-th triangular number t_k = k*(k+1)/2 has its base-12 digits contained in {1,5,7,11}. EXAMPLE 82 = 6X_12 is a term since the triangular number t=82*(82+1)/2 = 3403 = 1E77_12. MAPLE L:=[]: pd:={1, 5, 7, 11}: for w to 1 do for n from 1 to 10^6 do t:=n*(n+1)/2; lod:=convert(t, base, 12); sod:=convert(lod, set); if sod subset pd then L:=[op(L), [n, t]] fi; od od; L; MATHEMATICA fQ[n_] := Union@ Join[{1, 5, 7, 11}, IntegerDigits[n(n + 1)/2, 12]] == {1, 5, 7, 11}; Do[ If[fQ@n, AppendTo[lst, n]], {n, 10^8}] (* Robert G. Wilson v *) CROSSREFS Cf. A000217, A119033, A119034. Sequence in context: A217606 A002647 A306127 * A316309 A317264 A028874 Adjacent sequences: A118708 A118709 A118710 * A118712 A118713 A118714 KEYWORD nonn,base AUTHOR Walter Kehowski, May 24 2006 EXTENSIONS Edited and extended (a(23)-a(32)) by Robert G. Wilson v, Jun 20 2006 STATUS approved

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Last modified February 8 15:15 EST 2023. Contains 360149 sequences. (Running on oeis4.)