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 A240884 Semiprimes of the form C(n) + T(n) where C(n) and T(n) are the n-th cube and triangular numbers. 5
 33, 74, 237, 371, 1055, 1397, 10901, 12443, 30287, 39899, 55613, 80453, 207149, 303041, 360467, 407999, 639797, 1230821, 1650053, 2056511, 2695349, 2873441, 3454427, 3956873, 9823349, 10384103, 13680599, 15844877, 16419449, 20608499, 22705373, 26508143 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The n-th triangular number T(n) = n/2*(n+1). All the terms in the sequence, except a(2), are odd. Semiprimes (biprimes) in the sequence are product of two primes and simultaneously sum of n-th cube & triangular numbers. LINKS K. D. Bajpai, Table of n, a(n) for n = 1..6556 EXAMPLE a(1) = 33: 3^3 + 3/2*(3+1) = 33 = 3*11, which is product of two primes and hence semiprime. a(3) = 237: 6^3 + 6/2*(6+1) = 237 = 3*79, which is product of two primes and hence semiprime. MAPLE with(numtheory):KD:= proc() local a, b; a:=(n)^3+n/2*(n+1); b:=bigomega(a); if b=2 then RETURN (a); fi; end: seq(KD(), n=1..500); MATHEMATICA KD = {}; Do[t = n^3 + n/2*(n + 1); If[PrimeOmega[t] == 2, AppendTo[KD, t]], {n, 500}]; KD PROG (PARI) has(n)=if(n%2, isprime(n) && isprime(n^2+n\2+1), isprime(n/2) && isprime(2*n^2+n+1)) for(n=1, 1e4, if(has(n), print1(n^3+n*(n+1)/2", "))) \\ Charles R Greathouse IV, Aug 25 2014 CROSSREFS Cf. A001358, A005898, A046388. Sequence in context: A103046 A063868 A184417 * A049012 A137187 A134037 Adjacent sequences:  A240881 A240882 A240883 * A240885 A240886 A240887 KEYWORD nonn,easy AUTHOR K. D. Bajpai, Apr 14 2014 STATUS approved

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Last modified June 25 08:27 EDT 2021. Contains 345453 sequences. (Running on oeis4.)