A247159 Sum of divisors of even semiprimes.

7, 12, 18, 24, 36, 42, 54, 60, 72, 90, 96, 114, 126, 132, 144, 162, 180, 186, 204, 216, 222, 240, 252, 270, 294, 306, 312, 324, 330, 342, 384, 396, 414, 420, 450, 456, 474, 492, 504, 522, 540, 546, 576, 582, 594, 600, 636, 672, 684, 690, 702, 720, 726

Offset

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Formula

a(n) = sigma(2*prime(n)) = A000203(2*A000040(n)) = A000203(A100484(n)).

a(n) = 3*prime(n) + 3 for n > 1. - Charles R Greathouse IV, Nov 22 2014

Example

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.

Mathematica

DivisorSigma[1, #]&/@Select[Range[2, 500, 2], PrimeOmega[#]==2&] (* Harvey P. Dale, Jan 09 2015 *)
Join[{7}, Rest[3 Prime[Range[5000]] + 3]] (* Vincenzo Librandi, Jan 09 2018 *)

Prog

(PARI) v=3*apply(k->k+1, primes(100)); v[1]=7; v \\ Charles R Greathouse IV, Nov 22 2014
(Magma) [7] cat [3*NthPrime(n)+3: n in [2..60]]; // Vincenzo Librandi, Jan 09 2018

Crossrefs

Cf. A000040, A000203, A083681, A100484, A245685.

Sequence in context: A369357 A142337 A131912 * A022953 A030714 A190036

Adjacent sequences: A247156 A247157 A247158 * A247160 A247161 A247162

Keyword

nonn,easy

Author

Omar E. Pol, Nov 21 2014

Status

approved