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 A246907 #15 Sep 08 2022 08:46:09 %S A246907 1,2,4,8,16,64,128,2048,262144,17179869184,274877906944,8796093022208, %T A246907 36028797018963968 %N A246907 Numbers n such that sigma(n + sigma(n)) = 3n. %C A246907 Conjecture: for n >= 2; numbers n of the form 2^k such that 3*(2^k) - 1 is prime. The next terms: 18446744073709551616, 75557863725914323419136, 19807040628566084398385987584, … Sequence of numbers k: 1, 2, 3, 4, 6, 7, 11, 18, 34, 38, 43, 55, 64, 76, … Subsequence of A087370 (numbers n such that 3n - 1 is a prime). %C A246907 a(14) > 5*10^17. - _Hiroaki Yamanouchi_, Sep 11 2015 %e A246907 Number 16 is in sequence because sigma(16 + sigma(16)) = sigma(16 + 31) = sigma(47) = 48 = 3 * 16. %t A246907 Select[Range[300000], DivisorSigma[1, # + DivisorSigma[1, #]] == 3 # &] (* _Harvey P. Dale_, Jul 19 2015 *) %o A246907 (Magma) [n:n in[1..10000000] | SumOfDivisors(n+SumOfDivisors(n))eq 3*n] %o A246907 (PARI) %o A246907 for(n=1,10^7,if(sigma(n+sigma(n))==3*n,print1(n,", "))) \\ _Derek Orr_, Sep 07 2014 %Y A246907 Cf. A000203, A087370, A246906. %K A246907 nonn,more %O A246907 1,2 %A A246907 _Jaroslav Krizek_, Sep 07 2014 %E A246907 a(10)-a(13) from _Hiroaki Yamanouchi_, Sep 11 2015