Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #12 Apr 11 2022 12:57:14
%S 3,4,5,6,8,9,11,12,13,14,15,16,19,20,24,25,26,30,32,41,47,48,49,51,57,
%T 59,76,82,83,92,104,105,117,119,131,134,137,139,143,154,166,170,172,
%U 180,209,214,215,216,217,227,231,234,247,265,269,271,284,317,327,348
%N Numbers k such that A004394(k)-1 is prime.
%C First differs from A306588 at n=15.
%H Amiram Eldar, <a href="/A353300/b353300.txt">Table of n, a(n) for n = 1..314</a>
%e 3 is a term since A004394(3)-1 = 4-1 = 3 is prime.
%t s = {}; abm = 0; k = 0; Do[ab = DivisorSigma[-1, n]; If[ab > abm, abm = ab; k++; If[PrimeQ[n - 1], AppendTo[s, k]]], {n, 1, 10^6}]; s
%Y Cf. A004394, A072826, A306588, A353301, A353302.
%K nonn
%O 1,1
%A _Amiram Eldar_, Apr 10 2022