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A247789 Primes p such that sigma(2p-1) is a prime q. 2


%S 5,13,1201,1741,2521,5101,8581,14281,41761,139921,353641,595141,

%T 697381,1460341,1639861,3723721,3889261,6093541,7956061,11143921,

%U 14199121,33874681,46938361,51521401,56063461,62395621,67523821,90706981

%N Primes p such that sigma(2p-1) is a prime q.

%C Subsequence of A247820.

%C Corresponding values of primes q for a(n) are in A247836.

%e Prime 13 is in sequence because sigma(2*13-1) = sigma(25) = 31 (prime).

%o (MAGMA) [p: p in PrimesUpTo(50000000) | IsPrime(SumOfDivisors(2*p-1))]

%o (PARI) forprime(p=1,10^9,if(isprime(sigma(2*p-1)),print1(p,", "))) \\ _Derek Orr_, Sep 25 2014

%Y Cf. A000203, A008438, A000203, A247820.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Sep 24 2014

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Last modified August 12 12:09 EDT 2020. Contains 336439 sequences. (Running on oeis4.)