Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.
%I #28 Sep 08 2022 08:46:09
%S 7,12,18,24,36,42,54,60,72,90,96,114,126,132,144,162,180,186,204,216,
%T 222,240,252,270,294,306,312,324,330,342,384,396,414,420,450,456,474,
%U 492,504,522,540,546,576,582,594,600,636,672,684,690,702,720,726
%N Sum of divisors of even semiprimes.
%H Harvey P. Dale, <a href="/A247159/b247159.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = sigma(2*prime(n)) = A000203(2*A000040(n)) = A000203(A100484(n)).
%F a(n) = 3*prime(n) + 3 for n > 1. - _Charles R Greathouse IV_, Nov 22 2014
%e For n = 4 the 4th prime is 7 so the 4th even semiprime is 2*7 = 14. The sum of the divisors of 14 is 1 + 2 + 7 + 14 = 24, so a(4) = 24.
%t DivisorSigma[1,#]&/@Select[Range[2,500,2],PrimeOmega[#]==2&] (* _Harvey P. Dale_, Jan 09 2015 *)
%t Join[{7}, Rest[3 Prime[Range[5000]] + 3]] (* _Vincenzo Librandi_, Jan 09 2018 *)
%o (PARI) v=3*apply(k->k+1, primes(100)); v[1]=7; v \\ _Charles R Greathouse IV_, Nov 22 2014
%o (Magma) [7] cat [3*NthPrime(n)+3: n in [2..60]]; // _Vincenzo Librandi_, Jan 09 2018
%Y Cf. A000040, A000203, A083681, A100484, A245685.
%K nonn,easy
%O 1,1
%A _Omar E. Pol_, Nov 21 2014