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A074057
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a(n) = 2*phi(n-2)-(n-1).
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1
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0, -1, 0, -1, 2, -3, 4, -1, 2, -3, 8, -5, 10, -3, 0, -1, 14, -7, 16, -5, 2, -3, 20, -9, 14, -3, 8, -5, 26, -15, 28, -1, 6, -3, 12, -13, 34, -3, 8, -9, 38, -19, 40, -5, 2, -3, 44, -17, 34, -11, 12, -5, 50, -19, 24, -9, 14, -3, 56, -29, 58, -3, 8, -1, 30, -27, 64, -5, 18, -23, 68, -25, 70, -3, 4, -5, 42, -31, 76, -17, 26, -3, 80
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OFFSET
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3,5
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COMMENTS
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Conjecture : a(n)=0 if and only if n is a Fermat prime (A019434).
The conjecture appears to be false, a(83623937)=0 and 83623937 is not a Fermat number (A000215). See A232720 for sequence of n such that a(n)=0. - Michel Marcus, Nov 28 2013
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LINKS
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FORMULA
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a(n)=-1 if n is of the form 2^x+2;
a(n)=-3 if n is in a subsequence b(k) = 8, 12, 16, 24, 28, 36, 40, 48, 60, 64, 76, 84, 88, 96..( b(k) seems to be asymptotic to c*n*log(n) with c=2.28..).
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MATHEMATICA
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Table[2*EulerPhi[n-1]-n, {n, 2, 90}] (* Harvey P. Dale, May 22 2020 *)
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PROG
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(PARI) a(n) = 2*eulerphi(n-2) - (n-1); \\ Michel Marcus, Nov 28 2013
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CROSSREFS
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KEYWORD
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easy,sign
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AUTHOR
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STATUS
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approved
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