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%I #15 May 05 2017 12:37:42
%S 4,6,48,33592,44182,1918396,16975872,129518496,1098107800,23181002496,
%T 26187394752,36959761320,653669026776,1857670810368
%N Numbers n such that phi(n+2) - 2*phi(n+1) + phi(n) = -n.
%C The equation here is the difference equation (applied to phi) corresponding to the differential equation y" = -x (Hooke's law with constant = 1).
%C a(15) > 10^13. - _Giovanni Resta_, May 05 2017
%e Since phi(6) - 2*phi(5) + phi(4) = 2 - 2*4 + 2 = -4, 4 is a term of the sequence.
%t Select[ Range[1, 10^7], EulerPhi[ # + 2] - 2*EulerPhi[ # + 1] + EulerPhi[ # ] == - # & ]
%Y Cf. A000010 (phi).
%K nonn
%O 1,1
%A _Joseph L. Pe_, Dec 19 2001
%E More terms from _Robert G. Wilson v_, Dec 22 2001
%E a(7) and a(8) from _Harry J. Smith_, Feb 11 2010
%E a(9)-a(14) from _Giovanni Resta_, May 05 2017
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