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A247791 Primes p such that there is a prime q for which sigma(sigma(2q-1)) = p. 5
7, 131071, 524287 (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 there is prime q for which sigma(sigma(2q-1)) = A247954(q) = A000203(A000203(2q-1)) = A000203(A008438(q-1)) = A051027(2q-1) = p.

Corresponding values of primes q: 2, 28669, 126961, ... (A247790).

Conjecture: Subsequence of Mersenne primes.

Conjecture: the next term is 2305843009213693951 when 2305843009213693951 = sigma(sigma(2*500461553802019261-1)) where 500461553802019261 is prime (see comment of Hiroaki Yamanouchi in A247821). - Jaroslav Krizek, Oct 08 2014

LINKS

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

EXAMPLE

Prime 7 is in sequence because there is prime 2 such that sigma(sigma(2*2-1)) = sigma(sigma(3)) = sigma(4) = 7.

PROG

(MAGMA)[SumOfDivisors(SumOfDivisors(2*n-1)): n in [A247790(n)]

(MAGMA)[SumOfDivisors(SumOfDivisors(2*n-1)): n in[1..1000000] | IsPrime(SumOfDivisors(SumOfDivisors(2*n-1))) and IsPrime(n)]

(PARI)

forprime(p=1, 10^7, if(ispseudoprime(sigma(sigma(2*p-1))), print1(sigma(sigma(2*p-1)), ", "))) \\ Derek Orr, Sep 29 2014

CROSSREFS

Cf. A000203, A008438, A247790, A247820, A247821, A247822, A247823, A247954.

Sequence in context: A291906 A090769 A013842 * A243859 A297059 A306256

Adjacent sequences:  A247788 A247789 A247790 * A247792 A247793 A247794

KEYWORD

nonn,more,bref

AUTHOR

Jaroslav Krizek, Sep 28 2014

STATUS

approved

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Last modified January 22 13:41 EST 2020. Contains 331149 sequences. (Running on oeis4.)