17672631918domain: NAppears in sequencesa(n) = Sum_{d|n} phi(n/d)*C(2*d,d)/2.at n=18A081875a(n) = Sum_{1 <= x_1 <= x_2 <= ... <= x_n <= n} gcd(x_1, x_2, ... , x_n, n).at n=18A343517