Number of length n+2 0..2 arrays with at most two downsteps in every n consecutive neighbor pairs.

Terms

a(0) =27a(1) =81a(2) =243a(3) =694a(4) =1803a(5) =4248a(6) =9186a(7) =18483a(8) =35016a(9) =63060a(10) =108774a(11) =180801a(12) =290998a(13) =455313a(14) =694827a(15) =1036980a(16) =1517001a(17) =2179563a(18) =3080685a(19) =4289904a(20) =5892741a(21) =7993486a(22) =10718328a(23) =14218857a(24) =18675966a(25) =24304182a(26) =31356456a(27) =40129443a(28) =50969304a(29) =64278063