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 A256438 #20 Jul 16 2025 20:42:28 %S A256438 3,5,17,65,4097,65537,262145,1073741825,1152921504606846977, %T A256438 309485009821345068724781057,81129638414606681695789005144065, %U A256438 85070591730234615865843651857942052865 %N A256438 Numbers m such that sigma(sigma(m-1)) = 2*(m-1). %C A256438 Numbers k such that A051027(k-1) = 2*(k-1). %C A256438 Conjecture: numbers of the form 2^k+1 such that sigma(2^k) = prime p. %C A256438 Prime terms: 3, 5, 17, 65537, ... %C A256438 Supersequence of A249759. %F A256438 a(n) = A019279(n) + 1. - _Michel Marcus_, Feb 09 2020 %e A256438 17 is in the sequence because sigma(sigma(17-1)) = 32 = 2*(17-1). %p A256438 with(numtheory): A256438:=n->`if`(sigma(sigma(n-1)) = 2*(n-1), n, NULL): seq(A256438(n), n=2..10^5); # _Wesley Ivan Hurt_, Mar 30 2015 %t A256438 Select[Range@ 1000000, DivisorSigma[1, DivisorSigma[1, # - 1]] == 2 (# - 1) &] (* _Michael De Vlieger_, Mar 29 2015 *) %o A256438 (Magma) [n: n in[2..10000000] | SumOfDivisors(SumOfDivisors(n-1)) eq 2*(n-1)]; %o A256438 (PARI) isok(m) = sigma(sigma(m-1)) == 2*(m-1); \\ _Michel Marcus_, Feb 09 2020 %Y A256438 Cf. A000203, A051027, A019279, A249759. %K A256438 nonn,more %O A256438 1,1 %A A256438 _Jaroslav Krizek_, Mar 29 2015