a(n) = Sum_{1 <= x_1, x_2, x_3, x_4 <= n} n/gcd(x_1, x_2, x_3, x_4, n).

Terms

a(0) =1a(1) =31a(2) =241a(3) =991a(4) =3121a(5) =7471a(6) =16801a(7) =31711a(8) =58561a(9) =96751a(10) =161041a(11) =238831a(12) =371281a(13) =520831a(14) =752161a(15) =1014751a(16) =1419841a(17) =1815391a(18) =2476081a(19) =3092911a(20) =4049041a(21) =4992271a(22) =6436321a(23) =7642351a(24) =9753121a(25) =11509711a(26) =14230321a(27) =16649791a(28) =20511121a(29) =23316991