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 A247790 Primes p such that sigma(sigma(2p-1)) is a prime. 5
 2, 28669, 126961, 500461553802019261 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The next term, if it exists, must be greater than 5*10^7. Primes p such that A247954(p) = A000203(A000203(2p-1)) = A000203(A008438(p-1)) = A051027(2p-1) is a prime q. The corresponding values of the primes q are: 7, 131071, 524287, ... (A247791). Conjecture: the primes q are Mersenne primes (A000668). Conjecture: the next term is 500461553802019261 (see comment from Hiroaki Yamanouchi in A247821). - Jaroslav Krizek, Oct 08 2014 These are the primes in A247821. - M. F. Hasler, Oct 14 2014 No other terms up to 5*10^10. - Michel Marcus, Feb 11 2020 a(5) > 5*10^18. - Giovanni Resta, Feb 14 2020 LINKS EXAMPLE Prime 2 is in the sequence because sigma(sigma(2*2-1)) = sigma(sigma(3)) = sigma(4) = 7, i.e., prime. MAPLE with(numtheory): A247790:=n->`if`(isprime(n) and isprime(sigma(sigma(2*n-1))), n, NULL): seq(A247790(n), n=1..130000); # Wesley Ivan Hurt, Oct 17 2014 PROG (MAGMA) [p: p in PrimesUpTo(50000000) | IsPrime(SumOfDivisors(SumOfDivisors(2*p-1)))] (PARI) forprime(p=1, 10^7, if(ispseudoprime(sigma(sigma(2*p-1))), print1(p, ", "))) \\ Derek Orr, Sep 29 2014 CROSSREFS Cf. A000203, A008438, A247791, A247820, A247821, A247822, A247823, A247954. Sequence in context: A133857 A232869 A153912 * A233460 A324164 A047076 Adjacent sequences:  A247787 A247788 A247789 * A247791 A247792 A247793 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Sep 28 2014 EXTENSIONS a(4) from Giovanni Resta, Feb 14 2020 STATUS approved

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Last modified May 30 16:48 EDT 2020. Contains 334728 sequences. (Running on oeis4.)