a(n) = 3*C(n)-2, where C(n) = A000108(n).

Terms

a(0) =1a(1) =1a(2) =4a(3) =13a(4) =40a(5) =124a(6) =394a(7) =1285a(8) =4288a(9) =14584a(10) =50386a(11) =176356a(12) =624034a(13) =2228698a(14) =8023318a(15) =29084533a(16) =106073008a(17) =388934368a(18) =1432916098a(19) =5301789568a(20) =19692361258a(21) =73398801058a(22) =274447690918