a(n) = 4*a(n - 1) + 4*a(n - 2) for n>1, a(0)=2, a(1)=3.

Terms

a(0) =2a(1) =3a(2) =20a(3) =92a(4) =448a(5) =2160a(6) =10432a(7) =50368a(8) =243200a(9) =1174272a(10) =5669888a(11) =27376640a(12) =132186112a(13) =638251008a(14) =3081748480a(15) =14879997952a(16) =71846985728a(17) =346907934720