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 A270413 #15 Apr 13 2025 14:55:04 %S A270413 3,5,10,17,26,65,290,730,1682,2402,3482,4097,5042,7922,10202,15626, %T A270413 17162,27890,28562,29930,65537,83522,85850,146690,262145,279842, %U A270413 458330,491402,531442,552050,579122,597530,683930,703922,707282,734450,829922,1190282,1203410 %N A270413 Numbers m such that sigma(m-1) is a prime. %C A270413 Prime terms are in A249759. %C A270413 Corresponding values of sigma(n-1): 3, 7, 13, 31, 31, 127, 307, 1093, ... %C A270413 Conjecture: supersequence of A256438. %C A270413 Conjecture: 31 is the only prime p such that p = sigma(x-1) = sigma(y-1) for distinct numbers x and y; 31 = sigma(17-1) = sigma(26-1). %C A270413 Supersequence of A270414 and A270415. %F A270413 a(n) = A023194(n) + 1. %e A270413 17 is in the sequence because sigma(17-1) = sigma(16) = 31 (prime). %t A270413 Select[Range[10^6], PrimeQ@ DivisorSigma[1, # - 1] &] (* _Michael De Vlieger_, Mar 17 2016 *) %o A270413 (Magma) [n: n in [2..2000000] | IsPrime(SumOfDivisors(n-1))]; %o A270413 (PARI) isok(n) = isprime(sigma(n-1)); \\ _Michel Marcus_, Mar 17 2016 %Y A270413 Cf. A000203, A023194, A249759, A256438, A270414, A270415. %K A270413 nonn %O A270413 1,1 %A A270413 _Jaroslav Krizek_, Mar 16 2016