A249486 Nonprime numbers n such that sigma(n) + n is prime.

1, 4, 8, 16, 21, 27, 35, 36, 55, 57, 63, 64, 65, 75, 77, 85, 98, 100, 111, 119, 125, 128, 133, 143, 144, 155, 161, 171, 183, 189, 203, 205, 209, 215, 235, 237, 242, 243, 245, 253, 259, 275, 291, 301, 305, 323, 324, 333, 335, 338, 343, 351, 355, 365, 377, 391

Offset: 1

Author

Jaroslav Krizek, Nov 13 2014

Keywords

• nonn
• easy

Comments

Complement of A005384 (Sophie Germain primes) with respect to A078762 (numbers n such that n + sigma(n) is prime).

Examples

			Number 8 is in sequence because sigma(8)+8 = 15+8 = 23 (prime).
		

Crossrefs

Cf. A000040, A000203, A018252, A005384, A078762.

Programs

Magma

[n: n in[1..10000] | IsPrime(SumOfDivisors(n)+ n) and not IsPrime(n) ]

Maple

select(n -> not isprime(n) and isprime(n + numtheory:-sigma(n)), [$1..1000]); # Robert Israel, Nov 13 2014

Mathematica

Select[Range[500], PrimeQ[DivisorSigma[1, #] + #]&& !PrimeQ[#] &] (* Vincenzo Librandi, Nov 14 2014 *)

PARI

print1(1,", ");forcomposite(n=1,1000,if(isprime(sigma(n)+n),print1(n,", "))) \\ Derek Orr, Nov 13 2014