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Sum of positive integers k for k <= n and gcd(k,n) = gcd(k+1,n).
2

%I #10 Oct 20 2019 01:55:57

%S 1,0,1,0,6,0,15,0,12,0,45,0,66,0,21,0,120,0,153,0,50,0,231,0,180,0,

%T 117,0,378,0,435,0,144,0,255,0,630,0,209,0,780,0,861,0,198,0,1035,0,

%U 840,0,375,0,1326,0,729,0,476,0,1653,0,1770,0,465,0,1056,0,2145,0,714,0,2415

%N Sum of positive integers k for k <= n and gcd(k,n) = gcd(k+1,n).

%F a(n) = (n*(n-1)/2)*Sum_{d|n} mu(d)*tau(d)/d, n > 1.

%o (PARI) a(n) = if (n==1, 1, n*(n-1)/2*sumdiv(n, d, moebius(d)*numdiv(d)/d)) \\ _Michel Marcus_, Jun 17 2013

%Y Cf. A058026, A070555.

%K nonn

%O 1,5

%A _Vladeta Jovovic_, Apr 29 2002