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A203966
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Numbers n such that phi(n) divides n+1, where phi is Euler's totient function (A000010).
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17
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OFFSET
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1,2
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COMMENTS
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Except for a(2), all terms are odd. - Chai Wah Wu, Jun 06 2017
Since gcd(phi(n),n) = 1, all terms are squarefree. Then, for n = p1 * ... * pk with primes p1 < ... < pk, (n+1)/phi(n) is very close to p1/(p1-1)*...*pk/(p1-1). Since p/(p-1) is decreasing as p grows, having (n+1)/phi(n) = 3 implies that p1 >= 5 and further that n >= 2.4*10^56 is a product of at least 33 primes. Similarly, having (n+1)/phi(n) >= 4 implies that n >= 1.6*10^30 is a product of at least 21 primes. Hence, the terms of this sequence below 1.6*10^30 have (n+1)/phi(n) = 2 and thus coincide with A050474. - Max Alekseyev, Jan 30 2022
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LINKS
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EXAMPLE
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15 is in the sequence because phi(15) = 8, and 8 divides 16 = 15 + 1 evenly.
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MATHEMATICA
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Select[Range[100000], Divisible[#+1, EulerPhi[#]]&]
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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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