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 A113791 #12 Feb 16 2025 08:32:59 %S A113791 6,12,14,28,48,62,112,124,160,189,192,254,448,496,508,1984,2032,8128, %T A113791 12288,16382,28672,32764,126976,131056,196608,262142,458752,520192, %U A113791 524224,524284,786432,1048574,1835008,2031616,2097136,2097148,8126464 %N A113791 Let S(n)=Sigma(n)/2. Numbers n such that S(S(n))=n, 1/2-Sociable number of order 1 or 2. %C A113791 Almost all terms are of the form 2^m*M_k where M_k means the Mersenne prime 2^k-1. a(9)=2^5*5 and a(10)=3^3*7 are sporadic solutions. S(a(9))=a(10). %C A113791 All numbers of the form (M_j+1)/2 M_k, where M_j and M_k are Mersenne primes, are in this sequence. - _Robert G. Wilson v_ and _T. D. Noe_, Jan 21 2006 %H A113791 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SociableNumbers.html">Sociable Numbers</a> %t A113791 Select[ Range@8323071, DivisorSigma[1, DivisorSigma[1, # ]/2] == 2# &] (* _Robert G. Wilson v_ *) %K A113791 nonn %O A113791 1,1 %A A113791 _Yasutoshi Kohmoto_, Jan 21 2006 %E A113791 More terms from _Robert G. Wilson v_ and _T. D. Noe_, Jan 21 2006