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A271656
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Odd primes p such that phi(p+1) = phi(phi(p-2)+1).
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6
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5, 7, 17, 71, 257, 487, 761, 1151, 4327, 18287, 52721, 65537, 150767, 221471, 446597, 830861, 1002467, 2202787, 4978367, 6088337, 7534511, 9887651, 14278391, 21453101, 25032437, 31176863, 47290697, 51182837, 54264257, 80228717, 83623937, 138307877
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OFFSET
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1,1
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COMMENTS
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The first 4 known Fermat primes > 3 from A019434 are in the sequence.
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LINKS
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EXAMPLE
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257 is a term because phi(257+1) = phi(258) = 84 = phi(phi(257-2)+1) = phi(phi(255)+1) = phi(128+1) = phi(129).
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MATHEMATICA
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Select[Prime@ Range[10^6], EulerPhi[# + 1] == EulerPhi[EulerPhi[# - 2] + 1] &] (* Michael De Vlieger, Apr 11 2016 *)
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PROG
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(Magma) [n: n in [2..10^8] | IsPrime(n) and EulerPhi(n+1) eq EulerPhi(EulerPhi(n-2)+1)]
(PARI) is(n) = eulerphi(n+1)==eulerphi(eulerphi(n-2)+1)
forprime(p=3, , if(is(p), print1(p, ", "))) \\ Felix Fröhlich, Apr 23 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Definition adjusted and more terms added by Felix Fröhlich, Apr 23 2016
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STATUS
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approved
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