a(n) = 4*n! - 1.

Terms

a(0) =3a(1) =3a(2) =7a(3) =23a(4) =95a(5) =479a(6) =2879a(7) =20159a(8) =161279a(9) =1451519a(10) =14515199a(11) =159667199a(12) =1916006399a(13) =24908083199a(14) =348713164799