A308150 Numbers k such that sigma(k) mod k is prime, where sigma = A000203.

4, 8, 18, 20, 21, 27, 32, 35, 36, 39, 50, 55, 57, 63, 65, 77, 85, 98, 100, 104, 111, 115, 125, 128, 129, 155, 161, 171, 175, 185, 187, 189, 196, 201, 203, 205, 209, 221, 235, 237, 242, 245, 265, 275, 279, 291, 299, 305, 309, 319, 323, 324, 325, 327, 335, 338, 341, 365, 371, 377, 381, 385, 391

Offset: 1

Author

J. M. Bergot and Robert Israel, May 14 2019

Keywords

• nonn

Comments

Includes 1+A000668.

Examples

			a(3) = 18 is in the sequence because sigma(18) = 39, 39 == 3 (mod 18), and 3 is prime.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000203, A000668, A054024.

Includes A037020 and A075081.

Programs

Maple

select(n -> isprime(numtheory:-sigma(n) mod n), [$2..1000]);

PARI

isok(n) = isprime(sigma(n) % n); \\ Michel Marcus, May 15 2019