a(n) = L(3*n)/L(n), where L(n) = Lucas number.

Terms

a(0) =1a(1) =4a(2) =6a(3) =19a(4) =46a(5) =124a(6) =321a(7) =844a(8) =2206a(9) =5779a(10) =15126a(11) =39604a(12) =103681a(13) =271444a(14) =710646a(15) =1860499a(16) =4870846a(17) =12752044a(18) =33385281a(19) =87403804a(20) =228826126a(21) =599074579a(22) =1568397606a(23) =4106118244a(24) =10749957121a(25) =28143753124a(26) =73681302246a(27) =192900153619a(28) =505019158606