a(0)=4, a(1)=9, a(n) = 4a(n-1) - a(n-2).

Terms

a(0) =4a(1) =9a(2) =32a(3) =119a(4) =444a(5) =1657a(6) =6184a(7) =23079a(8) =86132a(9) =321449a(10) =1199664a(11) =4477207a(12) =16709164a(13) =62359449a(14) =232728632a(15) =868555079a(16) =3241491684a(17) =12097411657a(18) =45148154944a(19) =168495208119a(20) =628832677532