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A247790
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Primes p such that sigma(sigma(2p-1)) is a prime.
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5
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OFFSET
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1,1
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COMMENTS
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The next term, if it exists, must be greater than 5*10^7.
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LINKS
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EXAMPLE
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Prime 2 is in the sequence because sigma(sigma(2*2-1)) = sigma(sigma(3)) = sigma(4) = 7, i.e., prime.
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MAPLE
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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
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PROG
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(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
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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