a(n) = Sum_{x_1|n, x_2|n, x_3|n, x_4|n} gcd(x_1,x_2,x_3,x_4).

Terms

a(0) =1a(1) =17a(2) =18a(3) =99a(4) =20a(5) =306a(6) =22a(7) =373a(8) =119a(9) =340a(10) =26a(11) =1782a(12) =28a(13) =374a(14) =360a(15) =1115a(16) =32a(17) =2023a(18) =34a(19) =1980a(20) =396a(21) =442a(22) =38a(23) =6714a(24) =165a(25) =476a(26) =532a(27) =2178a(28) =44a(29) =6120