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A278919
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Numbers n such that phi(n-2) divides sigma(n-1)+1.
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0
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OFFSET
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1,1
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COMMENTS
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Supersequence of Fermat primes (A019434).
Conjecture: this sequence is finite.
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LINKS
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EXAMPLE
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3 is in this sequence because phi(1) divides sigma(2)+1; 1 divides 4.
4 is in this sequence because phi(2) divides sigma(3)+1; 1 divides 5.
5 is in this sequence because phi(3) divides sigma(4)+1; 2 divides 8.
17 is in this sequence because phi(15) divides sigma(16)+1; 8 divides 32.
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MATHEMATICA
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Select[Range[3, 66000], Divisible[DivisorSigma[1, (#-1)]+1, EulerPhi[#-2]]&] (* Ivan N. Ianakiev, Dec 05 2016 *)
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PROG
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(Magma) [3] cat [n: n in [4..10000000] | Denominator((SumOfDivisors(n-1)+1)/EulerPhi(n-2)) eq 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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EXTENSIONS
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STATUS
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approved
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