The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 4 17:59 EST 2020. Contains 338935 sequences. (Running on oeis4.)