A276238 - OEIS

%I #24 Aug 27 2019 05:25:23

%S 1,5,4097,7361,85073,658529,3999137,16777217,50281793,68544541,

%T 72281281,78202273,102931841,128459137,204786497,244622261,336392993,

%U 613750913,714155969,785432401,1008179297

%N Numbers k such that 2^sigma(k) == -1 (mod k).

%C Terms are 1, 5, 17*241, 17*433, 241*353, 17*38737, 353*11329, 97*257*673, ...

%C 1074702941, 1250862433, 1440721697, 1568001217, 1713613525, 2140345601, 2540788609, 3275604737, 4755026593, 5468121409, 5762812001, 6115556525, 6449655809, 6653504801, 9817079873, 14262957473, 15268309153, 25080521857, 50944294561, ... are also terms. - _Jinyuan Wang_, Aug 26 2019

%e 5 is a term because 2^sigma(5) = 64 and 64 == -1 (mod 5).

%o (PARI) is(k) = Mod(2, k)^sigma(k) == -1;

%Y Cf. A000079, A000203.

%K nonn,more

%O 1,2

%A _Altug Alkan_, Dec 12 2016

%E a(13)-a(21) from _Jinyuan Wang_, Aug 26 2019