A246858 Composite numbers k such that sigma(k + sigma(k)) = 2*sigma(k).

329, 413, 623, 869, 979, 1819, 2585, 3107, 3173, 3197, 3887, 4235, 4997, 5771, 6149, 6187, 6443, 7409, 8399, 8759, 14429, 15323, 18515, 19019, 21181, 21413, 23989, 26491, 29749, 30355, 31043, 32623, 34009, 34177, 39737, 47321, 47845, 51389, 53311, 56419

Offset: 1

Author

Jaroslav Krizek, Sep 05 2014

Keywords

• nonn

Comments

Complement of A005384 (Sophie Germain primes) with respect to A246857.

Examples

			Number 329 (with sigma(329) = 384) is in sequence because sigma(329 + sigma(329)) = sigma(713) = 768 = 2*384.
		

Crossrefs

Cf. A074400, A246456.

Cf. A005384, A246858.

Programs

Magma

[n:n in[1..1000] | SumOfDivisors(n+SumOfDivisors(n)) eq 2*SumOfDivisors(n) and not IsPrime(n)]

Mathematica

Select[Range[57000], And[CompositeQ[#], DivisorSigma[1, # + DivisorSigma[1, #]] == 2 DivisorSigma[1, #]] &] (* Michael De Vlieger, Aug 05 2021 *)

PARI

lista(nn) = {forcomposite(n=2, nn, if (sigma(n+sigma(n)) == 2*sigma(n), print1(n, ", ")););} \\ Michel Marcus, Sep 05 2014