Even Lucas numbers: a(n) = L(3*n).

Terms

a(0) =2a(1) =4a(2) =18a(3) =76a(4) =322a(5) =1364a(6) =5778a(7) =24476a(8) =103682a(9) =439204a(10) =1860498a(11) =7881196a(12) =33385282a(13) =141422324a(14) =599074578a(15) =2537720636a(16) =10749957122a(17) =45537549124a(18) =192900153618a(19) =817138163596