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 A122964 Semiprimes with triangular indices. 1
 4, 9, 15, 26, 39, 58, 85, 111, 134, 166, 205, 237, 291, 327, 382, 445, 497, 545, 623, 697, 766, 841, 921, 1003, 1114, 1195, 1285, 1379, 1486, 1622, 1718, 1837, 1943, 2071, 2195, 2329, 2483, 2605, 2787, 2935, 3073, 3214, 3379, 3554, 3742, 3909, 4105, 4286 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 FORMULA a(n) = semiprime(T(1)), where T(n) is the n-th triangular number. a(n) = A001358(A000217(n)) = A001358(n*(n+1)/2). EXAMPLE a(1) = semiprime(T(1)) = semiprime(1) = 4 = 2^2. a(2) = semiprime(T(2)) = semiprime(3) = 9 = 3^2. a(3) = semiprime(T(3)) = semiprime(6) = 15 = 3 * 5. a(4) = semiprime(T(4)) = semiprime(10) = 26 = 2 * 13. MATHEMATICA Module[{nn=3000, sem, tr, len}, sem=Select[Range[nn], PrimeOmega[#]==2&]; len = Length[sem]; tr=Table[If[OddQ[Sqrt[8n+1]], 1, 0], {n, len}]; Pick[ sem, tr, 1]](* Harvey P. Dale, Apr 24 2018 *) CROSSREFS Cf. A000217, A001358. Sequence in context: A244672 A056928 A225283 * A301254 A291318 A178379 Adjacent sequences: A122961 A122962 A122963 * A122965 A122966 A122967 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Oct 27 2006 EXTENSIONS a(24)-a(48) from Giovanni Resta, Jun 13 2016 STATUS approved

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Last modified December 8 05:22 EST 2022. Contains 358672 sequences. (Running on oeis4.)