A083681 Sum of divisors of semiprimes.

7, 12, 13, 18, 24, 24, 32, 36, 31, 42, 48, 54, 48, 60, 56, 72, 57, 72, 72, 80, 90, 96, 84, 96, 114, 96, 126, 108, 132, 120, 112, 128, 144, 120, 162, 152, 144, 180, 144, 133, 186, 168, 176, 160, 204, 192, 216, 168, 180, 222, 192, 240, 216, 192, 252, 183, 240, 270, 248

Offset

1,1

Comments

Contains all products of distinct terms of A008864 contributed by the squarefree semiprimes and all terms of A060800 contributed by the squared primes: 7 = A060800(1), 12 = A008864(1)*A008864(2), 13 = A060800(2), 18=A008864(1)*A008864(3) etc. - R. J. Mathar, Mar 15 2018

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Formula

a(n) = A000203(A001358(n)) = 1 + A020639(n) + A057427(A006530(n)-A020639(n))*A006530(n) + A020639(n)*A006530(n). - Reinhard Zumkeller, Jun 16 2003

Example

a(2) = 12 because the sum of divisors of the 2nd semiprime, i.e. 6, is 1+2+3+6 = 12.

Mathematica

DivisorSigma[1, Select[Range[200], PrimeOmega[#] == 2 &]] (* Amiram Eldar, Nov 24 2020 *)

Crossrefs

Cf. A000203, A001358, A006530, A008864, A020639, A060800.

Sequence in context: A223423 A274334 A328414 * A178660 A048653 A205807

Adjacent sequences: A083678 A083679 A083680 * A083682 A083683 A083684

Keyword

easy,nonn

Author

Shyam Sunder Gupta, Jun 15 2003

Status

approved