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 A207339 Triangular numbers T from A000217 such that (4*T+1)/5 is prime. 6
 6, 21, 36, 66, 91, 136, 171, 351, 496, 561, 741, 946, 1176, 1326, 1596, 2016, 2346, 2701, 2926, 3321, 3486, 4851, 6216, 6441, 7626, 8646, 8911, 9591, 10011, 10296, 11026, 11476, 13041, 15051, 16471, 16836, 17391, 18336, 19701, 21736 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The corresponding primes are gven in A207337, where also equivalent formulations are found. The indices of these triangular numbers are given by (A002733(n)-1)/2. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 FORMULA a(n) = T(K(n)):= A000217(K(n)) with K(n)=(m(n)-1)/2, and m(n) given in A002733(n). EXAMPLE a(3) = 36 = T((17-1)/2) = T(8)=A000217(8). (4*36+1)/5 = 29 = A207337(3). MATHEMATICA Select[Accumulate[Range[300]], PrimeQ[(4#+1)/5]&] (* Harvey P. Dale, Sep 18 2019 *) CROSSREFS Cf. A207337, A129307, A027862, A002731. Sequence in context: A047717 A089982 A151943 * A284988 A056488 A031042 Adjacent sequences:  A207336 A207337 A207338 * A207340 A207341 A207342 KEYWORD nonn AUTHOR Wolfdieter Lang, Feb 27 2012 STATUS approved

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Last modified January 20 01:01 EST 2022. Contains 350467 sequences. (Running on oeis4.)