a(n) = A094538(n)/3.

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =11a(5) =33a(6) =105a(7) =315a(8) =961a(9) =2883a(10) =8697a(11) =26091a(12) =78411a(13) =235233a(14) =706113a(15) =2118339a(16) =6356243a(17) =19068729a(18) =57209865a(19) =171629595a(20) =514899771a(21) =1544699313a(22) =4634130897a(23) =13902392691a(24) =41707276809a(25) =125121830427a(26) =375365787489