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

Terms

a(0) =1a(1) =6a(2) =17a(3) =42a(4) =74a(5) =153a(6) =216a(7) =379a(8) =531a(9) =809a(10) =1011a(11) =1605a(12) =1832a(13) =2626a(14) =3268a(15) =4304a(16) =4861a(17) =6798a(18) =7333a(19) =9878a(20) =11148a(21) =13711a(22) =14972a(23) =19985a(24) =20775a(25) =25643a(26) =28503a(27) =34517a(28) =35988a(29) =46162