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A249761
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Primes p such that there is prime q with sigma(q-1) = p.
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2
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OFFSET
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1,1
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COMMENTS
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Corresponding values of primes q are in A249759.
Conjectures: 1) sequence is finite; 2) (a(n)+3)/2 is a Fermat prime (A019434); 3) a(n) is subsequence of Mersenne primes (A000668); 4) a(n) = 2*A249759(n)-3.
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LINKS
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EXAMPLE
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Prime 31 is in the sequence because there is prime 17 with sigma(17-1) = sigma(16) = 31.
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PROG
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(Magma) [a: p in PrimesUpTo(5000000) | IsPrime(a) where a is SumOfDivisors(p-1)]
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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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STATUS
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approved
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