

A247790


Primes p such that sigma(sigma(2p1)) is a prime.


5




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(2p1)) = A000203(A008438(p1)) = A051027(2p1) 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 of Hiroaki Yamanouchi in A247821).  Jaroslav Krizek, Oct 08 2014
These are the primes in A247821.  M. F. Hasler, Oct 14 2014


LINKS

Table of n, a(n) for n=1..3.


EXAMPLE

Prime 2 is in the sequence because sigma(sigma(2*21)) = sigma(sigma(3)) = sigma(4) = 7, i.e., prime.


MAPLE

with(numtheory): A247790:=n>`if`(isprime(n) and isprime(sigma(sigma(2*n1))), 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*p1)))]
(PARI) forprime(p=1, 10^7, if(ispseudoprime(sigma(sigma(2*p1))), 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,bref


AUTHOR

Jaroslav Krizek, Sep 28 2014


STATUS

approved



