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A232720
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Numbers such that 2*phi(n-2) = n-1.
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4
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OFFSET
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1,1
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COMMENTS
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Note that the first 5 terms are Fermat primes (A019434), while a(6) is prime but is not a Fermat number, and a(7) is (F5 from A000215) but is not prime. Note also that F6, F7, F8 and F9 do not belong to this sequence.
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LINKS
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FORMULA
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PROG
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(PARI) is_A232720(n) = 2*eulerphi(n-2) == n-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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