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

Terms

a(0) =4a(1) =1a(2) =13a(3) =13a(4) =73a(5) =121a(6) =481a(7) =1009a(8) =3361a(9) =7969a(10) =24193a(11) =61249a(12) =177025a(13) =464257a(14) =1307137a(15) =3493633a(16) =9699841a(17) =26190337a(18) =72173569a(19) =195941377a(20) =537802753a(21) =1464342529a(22) =4010582017a(23) =10937266177a(24) =29920862209a(25) =81665925121a(26) =223274237953a(27) =609678999553