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%I #14 Aug 15 2021 02:16:28
%S 1,2,4,12,17,55,57,80,195,211,233,602,694,1319,2726,26312,71173,
%T 101457,145789,165710,299228,483888,718738,1757846,4206121,9518456,
%U 15505388,15885915,15929230,26445656,28665696,37875137,147389152,218849960,430031707,507418131
%N Numbers k that divide the alternating sum phi(1) - phi(2) + phi(3) - phi(4) + ... + ((-1)^(k+1))*phi(k).
%C phi(1) - phi(2) + phi(3) - phi(4) = 1 - 1 + 2 - 2 = 0, which is divisible by 4, so 4 is a term of the sequence.
%t s = 0; Do[s = s + (-1)^(i + 1) * EulerPhi[i]; If[ Mod[s, i] == 0, Print[i]], {i, 1, 10^7}]
%o (PARI) {a067929(m)=local(s,n); s=0; for(n=1,m, if(n%2==0,s=s-eulerphi(n),s=s+eulerphi(n)); if(s%n==0,print1(n,",")))}
%Y Cf. A000010, A068773.
%K nonn
%O 1,2
%A _Joseph L. Pe_, Feb 22 2002
%E Edited and extended by _Robert G. Wilson v_ and _Klaus Brockhaus_, Feb 27 2002
%E a(27)-a(36) from _Donovan Johnson_, Jul 26 2011
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