Number of order-3 ribbon tilings for a 3 X n strip.

Terms

a(0) =1a(1) =2a(2) =6a(3) =12a(4) =26a(5) =61a(6) =134a(7) =297a(8) =669a(9) =1490a(10) =3321a(11) =7426a(12) =16573a(13) =36986a(14) =82594a(15) =184382a(16) =411599a(17) =918931a(18) =2051485a(19) =4579826a(20) =10224441a(21) =22825825a(22) =50957971a(23) =113762629a(24) =253972415a(25) =566987102a(26) =1265785592a(27) =2825836529a(28) =6308612558a(29) =14083828826