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A256510
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Primes p such that phi(p-2) = phi(p-1).
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1
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3, 5, 17, 257, 977, 3257, 5189, 11717, 13367, 22937, 65537, 307397, 491537, 589409, 983777, 1659587, 2822717, 3137357, 5577827, 6475457, 7378373, 8698097, 10798727, 32235737, 37797437, 39220127, 39285437, 51555137, 52077197, 56992553, 63767927, 70075997
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OFFSET
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1,1
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COMMENTS
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First 5 Fermat primes from A019434 are terms of this sequence.
a(2) = 5 is only term of a(n) such that a(n) - 2 is a prime q, i.e., prime 3 is only prime q such that phi(q) = phi(q+1).
If there are any other Fermat primes, they will not be in the sequence. - Robert Israel, Mar 31 2015
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LINKS
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EXAMPLE
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Prime 17 is in the sequence because phi(15) = phi(16) = 8.
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MAPLE
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MATHEMATICA
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Select[Prime@ Range@ 100000, EulerPhi[# - 2] == EulerPhi[# - 1] &] (* Michael De Vlieger, Mar 31 2015 *)
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PROG
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(Magma) [n: n in [3..10^7] | IsPrime(n) and EulerPhi(n-2) eq EulerPhi(n-1)]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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