a(n) = 2*a(n-1) + 4*a(n-2) - 2*a(n-3) with initial terms 1, 3, 9.

Terms

a(0) =1a(1) =3a(2) =9a(3) =28a(4) =86a(5) =266a(6) =820a(7) =2532a(8) =7812a(9) =24112a(10) =74408a(11) =229640a(12) =708688a(13) =2187120a(14) =6749712a(15) =20830528a(16) =64285664a(17) =198394016a(18) =612269632a(19) =1889544000a(20) =5831378496a(21) =17996393728a(22) =55539213440a(23) =171401244800a(24) =528966555904