a(n) = ((5+sqrt(3))*(1+sqrt(3))^n + (5-sqrt(3))*(1-sqrt(3))^n)/2.

Terms

a(0) =5a(1) =8a(2) =26a(3) =68a(4) =188a(5) =512a(6) =1400a(7) =3824a(8) =10448a(9) =28544a(10) =77984a(11) =213056a(12) =582080a(13) =1590272a(14) =4344704a(15) =11869952a(16) =32429312a(17) =88598528a(18) =242055680a(19) =661308416a(20) =1806728192a(21) =4936073216a(22) =13485602816a(23) =36843352064a(24) =100657909760a(25) =275002523648