a(n) = Lucas(2*n) + 2^(n + 1) + 1.

Terms

a(0) =8a(1) =16a(2) =35a(3) =80a(4) =188a(5) =451a(6) =1100a(7) =2720a(8) =6803a(9) =17176a(10) =43700a(11) =111875a(12) =287828a(13) =743416a(14) =1926035a(15) =5001920a(16) =13014188a(17) =33909571a(18) =88452380a(19) =230923280a(20) =603268883a(21) =1576786216a(22) =4122895460a(23) =10783511555a(24) =28210861988a(25) =73815519976a(26) =193168589075a(27) =505556029520