a(n) = 6*Lucas(2n) - Fibonacci(2n+2).

Terms

a(0) =11a(1) =15a(2) =34a(3) =87a(4) =227a(5) =594a(6) =1555a(7) =4071a(8) =10658a(9) =27903a(10) =73051a(11) =191250a(12) =500699a(13) =1310847a(14) =3431842a(15) =8984679a(16) =23522195a(17) =61581906a(18) =161223523a(19) =422088663a(20) =1105042466a(21) =2893038735a(22) =7574073739a(23) =19829182482a(24) =51913473707a(25) =135911238639