a(1) = 1; for n > 1, a(n) = 2*a(n-1) + lcm(a(n-1),n).

Terms

a(0) =1a(1) =4a(2) =20a(3) =60a(4) =180a(5) =540a(6) =4860a(7) =19440a(8) =58320a(9) =174960a(10) =2274480a(11) =6823440a(12) =20470320a(13) =184232880a(14) =552698640a(15) =1658095920a(16) =31503822480a(17) =94511467440a(18) =283534402320a(19) =850603206960