A068773 - OEIS

%I

%S 1,0,2,0,4,2,8,4,10,6,16,12,24,18,26,18,34,28,46,38,50,40,62,54,74,62,

%T 80,68,96,88,118,102,122,106,130,118,154,136,160,144,184,172,214,194,

%U 218,196,242,226,268,248,280,256,308,290,330,306,342,314,372,356

%N Alternating sum phi(1) - phi(2) + phi(3) - phi(4) + ... + ((-1)^(n+1))*phi(n).

%F a(n) = Sum_{k=1..n} (-1)^(k+1)*phi(k).

%e a(3) = phi(1) - phi(2) + phi(3) = 1 - 1 + 2 = 2.

%o (PARI) a068773(m)=local(s,n); s=0; for(n=1,m, if(n%2==0,s=s-eulerphi(n),s=s+eulerphi(n)); print1(s,","))

%Y Cf. A000010, A067929.

%K easy,nonn

%O 1,3

%A _Klaus Brockhaus_, Feb 28 2002