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 A247838 Numbers n such that sigma(sigma(n)) is prime. 4
 3, 2667, 3937, 57337, 172011, 253921, 677207307, 1073602561, 732959441001382539, 750688035198863979, 1000923107604038521, 1108158528150703969 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that A051027(n) is a prime p. Prime 3 is the only prime p such that sigma(sigma(p)) is a prime q. Conjecture: Subsequence of A046528 (numbers that are a product of distinct Mersenne primes). Corresponding values of primes p: 7, 8191, 8191, 131071, 524287, 524287, ... (A247822). Conjecture: values of primes p is equal to Mersenne primes (A000668). 732959441001382539, 750688035198863979, 1000923107604038521, 1108158528150703969 and 196751176038481899983340171 are terms. - Jaroslav Krizek, Mar 25 2015 a(9) > 10^10. - Michel Marcus, Feb 13 2020 a(13) > 10^19. - Giovanni Resta, Feb 14 2020 LINKS FORMULA a(n) = 2*A247821(n)-1. EXAMPLE 2667 is a term because sigma(sigma(2667)) = sigma(4096) = 8191 (i.e., prime). MAPLE with(numtheory): A247838:=n->`if`(isprime(sigma(sigma(n))), n, NULL): seq(A247838(n), n=1..10^5); # Wesley Ivan Hurt, Oct 02 2014 PROG (Magma) [n: n in [1..10000000] | IsPrime(SumOfDivisors(SumOfDivisors(n)))] (PARI) isok(n) = isprime(sigma(sigma(n))); \\ Michel Marcus, Oct 01 2014 CROSSREFS Cf. A000203, A023194, A063103, A000668, A046528, A051027, A247821, A247822, A247954. Sequence in context: A171361 A203687 A034316 * A003534 A202520 A281928 Adjacent sequences: A247835 A247836 A247837 * A247839 A247840 A247841 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Sep 28 2014 EXTENSIONS a(7)-a(8) from Michel Marcus, Oct 02 2014 a(9)-a(12) from Giovanni Resta, Feb 14 2020 STATUS approved

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Last modified January 27 18:55 EST 2023. Contains 359845 sequences. (Running on oeis4.)