16510911domain: NAppears in sequencesa(n) = Sum_{d|n} (-1)^(n/d+1)*d^6.at n=15A284927a(n) = Sum_{1 <= x_1, x_2, x_3, x_4, x_5 <= n} n/gcd(x_1, x_2, x_3, x_4, x_5, n).at n=15A371878