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 A208294 Triangular numbers T from A000217 such that (4*T+1)/13 is prime. 3
 55, 120, 1485, 1953, 2278, 2850, 3240, 6555, 9870, 10585, 16110, 17020, 18528, 34453, 36585, 37950, 47895, 49455, 52003, 56280, 57970, 70125, 72010, 80200, 96580, 110685, 127260, 129795, 133903, 165025, 169653, 172578, 244650 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The corresponding primes are gven in A208292, where equivalent formulations are found. The indices of these triangular numbers are given by (A208293(n)-1)/2. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..5000 FORMULA a(n) = T(K(n)):= A000217(K(n)) with K(n)=(A208293(n)-1)/2. EXAMPLE a(2) = 120. m(2)= 31: 120 = T((31-1)/2) = T(15)=A000217(15). (4*120+1)/13 = 37 = A208292(2). MATHEMATICA tri = # (# + 1)/2 & /@ Range@ 1000; Select[ tri, PrimeQ[(4 # + 1)/13] &] (* Robert G. Wilson v, Feb 28 2012 *) CROSSREFS Cf. A208292. A208293, A207337, A207339, A129307, A027862, A002731. Sequence in context: A044623 A063345 A080701 * A039442 A063324 A046156 Adjacent sequences:  A208291 A208292 A208293 * A208295 A208296 A208297 KEYWORD nonn AUTHOR Wolfdieter Lang, Feb 27 2012 STATUS approved

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Last modified August 2 16:46 EDT 2021. Contains 346428 sequences. (Running on oeis4.)