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

Author

Omar E. Pol, Nov 21 2014

Keywords

• nonn
• easy

Examples

			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.
		

Links

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

Crossrefs

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

Programs

Magma

[7] cat [3*NthPrime(n)+3: n in [2..60]]; // Vincenzo Librandi, Jan 09 2018

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 *)

PARI

v=3*apply(k->k+1, primes(100)); v[1]=7; v \\ Charles R Greathouse IV, Nov 22 2014

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