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 A249486 Nonprime numbers n such that sigma(n) + n is prime. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Complement of A005384 (Sophie Germain primes) with respect to A078762 (numbers n such that n + sigma(n) is prime). LINKS Table of n, a(n) for n=1..56. EXAMPLE Number 8 is in sequence because sigma(8)+8 = 15+8 = 23 (prime). 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 *) PROG (Magma) [n: n in[1..10000] | IsPrime(SumOfDivisors(n)+ n) and not IsPrime(n) ] (PARI) print1(1, ", "); forcomposite(n=1, 1000, if(isprime(sigma(n)+n), print1(n, ", "))) \\ Derek Orr, Nov 13 2014 CROSSREFS Cf. A000040, A000203, A018252, A005384, A078762. Sequence in context: A335792 A312807 A312808 * A129370 A344485 A212009 Adjacent sequences: A249483 A249484 A249485 * A249487 A249488 A249489 KEYWORD nonn,easy AUTHOR Jaroslav Krizek, Nov 13 2014 STATUS approved

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Last modified May 29 04:26 EDT 2024. Contains 372921 sequences. (Running on oeis4.)