Number of length n+1 0..2 arrays with at most two downsteps in every n consecutive neighbor pairs.

Terms

a(0) =9a(1) =27a(2) =81a(3) =237a(4) =651a(5) =1647a(6) =3834a(7) =8272a(8) =16698a(9) =31824a(10) =57720a(11) =100296a(12) =167898a(13) =272034a(14) =428247a(15) =657153a(16) =985663a(17) =1448409a(18) =2089395a(19) =2963895a(20) =4140621a(21) =5704185a(22) =7757880a(23) =10426806a(24) =13861368a(25) =18241174a(26) =23779362a(27) =30727386a(28) =39380292a(29) =50082516