a(n) = Sum_{1 <= i <= j <= k <= m <= n} gcd(i,j,k,m).

Terms

a(0) =1a(1) =6a(2) =18a(3) =44a(4) =83a(5) =159a(6) =249a(7) =401a(8) =592a(9) =867a(10) =1163a(11) =1655a(12) =2122a(13) =2796a(14) =3594a(15) =4594a(16) =5579a(17) =7046a(18) =8394a(19) =10328a(20) =12339a(21) =14699a(22) =17021a(23) =20441a(24) =23526a(25) =27317a(26) =31379a(27) =36323a(28) =40846a(29) =47300