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A247159 Sum of divisors of even semiprimes. 3
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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
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
Sequence in context: A369357 A142337 A131912 * A022953 A030714 A190036
KEYWORD
nonn,easy
AUTHOR
Omar E. Pol, Nov 21 2014
STATUS
approved

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Last modified July 23 23:47 EDT 2024. Contains 374575 sequences. (Running on oeis4.)