A276238 Numbers k such that 2^sigma(k) == -1 (mod k).

1, 5, 4097, 7361, 85073, 658529, 3999137, 16777217, 50281793, 68544541, 72281281, 78202273, 102931841, 128459137, 204786497, 244622261, 336392993, 613750913, 714155969, 785432401, 1008179297

Offset: 1

Author

Altug Alkan, Dec 12 2016

Keywords

• nonn
• more

Comments

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

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

Examples

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

Crossrefs

Cf. A000079, A000203.

Programs

PARI

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

Extensions

a(13)-a(21) from Jinyuan Wang, Aug 26 2019