a(n) = (1/4)*(5*(n-1)*P(n)+n*P(n-1)) where P() are the Pell numbers A000129.

Terms

a(0) =0a(1) =3a(2) =14a(3) =50a(4) =160a(5) =481a(6) =1390a(7) =3908a(8) =10768a(9) =29215a(10) =78302a(11) =207798a(12) =546960a(13) =1429821a(14) =3715870a(15) =9608200a(16) =24734816a(17) =63429563a(18) =162099950a(19) =412993210a(20) =1049319872a(21) =2659463705a(22) =6725142286a(23) =16971369036a(24) =42748016880a(25) =107489296599a(26) =269850534206a(27) =676460107070