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A258454
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Numbers n such that phi(n) = 2*phi(n-2).
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0
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3, 4, 5, 8, 17, 32, 257, 512, 527, 992, 1952, 2522, 5252, 6512, 7412, 10376, 23432, 23717, 26732, 27302, 35114, 36632, 37442, 45872, 47027, 49022, 51092, 65537, 78899, 84242, 92432, 98432, 98672, 114767, 115292, 131072, 227222, 231167, 240977, 328352, 369272
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OFFSET
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1,1
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COMMENTS
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Conjecture: a prime p is in the sequence iff p is a Fermat prime (A019434).
This is not correct: the first non-Fermat prime term is 83623937 = 2^18*11*29 + 1. - Joerg Arndt, Oct 11 2015
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LINKS
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EXAMPLE
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phi(32) = 16 = 2*phi(30) = 2*8, so 32 is in the sequence.
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MATHEMATICA
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Select[Range@ 400000, EulerPhi@ # == 2 EulerPhi[# - 2] &] (* Michael De Vlieger, Sep 25 2015 *)
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PROG
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(Magma) [n: n in [3..10000000] | EulerPhi(n) eq 2*EulerPhi(n-2)]
(PARI) for(n=1, 1e6, if(eulerphi(n) == 2*eulerphi(n-2), print1(n", "))); \\ Altug Alkan, Sep 26 2015
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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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