a(n) = n*((2+sqrt(3))^n + (2-sqrt(3))^n)/2.

Terms

a(0) =0a(1) =2a(2) =14a(3) =78a(4) =388a(5) =1810a(6) =8106a(7) =35294a(8) =150536a(9) =632034a(10) =2620870a(11) =10759342a(12) =43804812a(13) =177105266a(14) =711809378a(15) =2846259390a(16) =11330543632a(17) =44929049794a(18) =177540878718a(19) =699402223118