a(n) = Sum_{1 <= x_1, x_2, x_3 <= n} gcd(x_1, x_2, x_3, n)^3.

Terms

a(0) =1a(1) =15a(2) =53a(3) =176a(4) =249a(5) =795a(6) =685a(7) =1856a(8) =2133a(9) =3735a(10) =2661a(11) =9328a(12) =4393a(13) =10275a(14) =13197a(15) =18432a(16) =9825a(17) =31995a(18) =13717a(19) =43824a(20) =36305a(21) =39915a(22) =24333a(23) =98368a(24) =46625a(25) =65895a(26) =76545a(27) =120560a(28) =48777a(29) =197955