login

Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.

Numbers k such that A004394(k)-1 is prime.
3

%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