a(n) = (1/2)*((n+2)*P(n-1)+(5*n+1)*P(n)) where P() = A000129 are the Pell numbers.

Terms

a(0) =3a(1) =13a(2) =45a(3) =141a(4) =419a(5) =1201a(6) =3357a(7) =9209a(8) =24899a(9) =66549a(10) =176205a(11) =462917a(12) =1208163a(13) =3135449a(14) =8097597a(15) =20823921a(16) =53350531a(17) =136228445a(18) =346819437a(19) =880594813a(20) =2230476067a(21) =5637208449a(22) =14218682973a(23) =35797815913a(24) =89974587843a(25) =225790779205a(26) =565803994509