Multiply by 1, add 1, multiply by 2, add 2, etc.; start with 3.

Terms

a(0) =3a(1) =3a(2) =4a(3) =8a(4) =10a(5) =30a(6) =33a(7) =132a(8) =136a(9) =680a(10) =685a(11) =4110a(12) =4116a(13) =28812a(14) =28819a(15) =230552a(16) =230560a(17) =2075040a(18) =2075049a(19) =20750490a(20) =20750500a(21) =228255500a(22) =228255511a(23) =2739066132a(24) =2739066144a(25) =35607859872a(26) =35607859885a(27) =498510038390a(28) =498510038404