This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A083681 #14 Nov 24 2020 10:24:41 %S A083681 7,12,13,18,24,24,32,36,31,42,48,54,48,60,56,72,57,72,72,80,90,96,84, %T A083681 96,114,96,126,108,132,120,112,128,144,120,162,152,144,180,144,133, %U A083681 186,168,176,160,204,192,216,168,180,222,192,240,216,192,252,183,240,270,248 %N A083681 Sum of divisors of semiprimes. %C A083681 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 %H A083681 Amiram Eldar, <a href="/A083681/b083681.txt">Table of n, a(n) for n = 1..10000</a> %F A083681 a(n) = A000203(A001358(n)) = 1 + A020639(n) + A057427(A006530(n)-A020639(n))*A006530(n) + A020639(n)*A006530(n). - _Reinhard Zumkeller_, Jun 16 2003 %e A083681 a(2) = 12 because the sum of divisors of the 2nd semiprime, i.e. 6, is 1+2+3+6 = 12. %t A083681 DivisorSigma[1, Select[Range[200], PrimeOmega[#] == 2 &]] (* _Amiram Eldar_, Nov 24 2020 *) %Y A083681 Cf. A000203, A001358, A006530, A008864, A020639, A060800. %K A083681 easy,nonn %O A083681 1,1 %A A083681 _Shyam Sunder Gupta_, Jun 15 2003