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(phi(n+1)-phi(n))/2.
1

%I #14 Sep 08 2022 08:45:01

%S 0,1,-1,2,-1,1,-1,3,-3,4,-3,1,0,4,-5,6,-5,2,-1,6,-7,6,-4,3,-3,8,-10,

%T 11,-7,2,-2,4,-6,12,-9,3,-4,12,-14,15,-11,2,-1,12,-15,13,-11,6,-4,14,

%U -17,11,-8,6,-4,15,-21,22,-15,3,-2,8,-14,23,-17,6,-10,23,-23,24,-18,2,-2,12,-18,27,-23,11

%N (phi(n+1)-phi(n))/2.

%H T. D. Noe, <a href="/A057001/b057001.txt">Table of n, a(n) for n=3..10000</a>

%t Table[(EulerPhi[n + 1] - EulerPhi[n])/2, {n, 3, 50}] (* _Daniel Jolly_, Nov 02 2014 *)

%o (Magma) [(EulerPhi(n + 1) - EulerPhi(n))/2: n in [3..100]]; // _Vincenzo Librandi_, Nov 03 2014

%Y Cf. A000010.

%K sign

%O 3,4

%A _N. J. A. Sloane_, Sep 09 2000