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 A137281 Numbers k such that T(k) is not divisible by T(i), 1 < i < k, where T(k) = k-th triangular number A000217(k). 5
 2, 4, 7, 10, 13, 16, 22, 25, 28, 31, 34, 37, 43, 46, 49, 52, 58, 61, 67, 70, 73, 76, 82, 85, 88, 94, 97, 103, 106, 118, 121, 127, 130, 133, 136, 142, 145, 148, 151, 157, 163, 166, 169, 172, 178, 187, 190, 193, 196, 202, 205, 208, 211, 214, 217, 226, 229, 232, 238, 241 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms > 5 in A005383 are here. - Zak Seidov, Jun 20 2013 All terms except 2 are congruent to 1 (mod 3). This is required for 3 not to be a divisor of T(n). - Franklin T. Adams-Watters, Dec 10 2019 LINKS T. D. Noe and Zak Seidov, Table of n, a(n) for n = 1..10000 (first 1000 terms from T. D. Noe) FORMULA n such that A076982(n) = 2. - T. D. Noe, Apr 12 2011 A000217(a(n)) = A226863(n). - Zak Seidov, Jun 20 2013 EXAMPLE T(4)=10 is not divisible by lesser T's 3, 6; T(7)=28 is not divisible by lesser T's 3, 6, 10, 15, 21. MATHEMATICA nn = 241; tri = Table[n*(n+1)/2, {n, nn}]; Select[Range[2, nn], ! MemberQ[Mod[tri[[#]], Take[tri, {2, # - 1}]], 0] &] (* T. D. Noe, Apr 12 2011 *) CROSSREFS Cf. A000217, A005383, A226863. Sequence in context: A194228 A194236 A127762 * A287420 A003067 A130243 Adjacent sequences: A137278 A137279 A137280 * A137282 A137283 A137284 KEYWORD nonn AUTHOR Zak Seidov, Mar 14 2008 STATUS approved

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Last modified September 30 21:59 EDT 2023. Contains 365812 sequences. (Running on oeis4.)