Number of zig-zag paths from top to bottom of a rectangle of width 9 with n rows.

Terms

a(0) =9a(1) =16a(2) =30a(3) =56a(4) =106a(5) =200a(6) =380a(7) =720a(8) =1370a(9) =2600a(10) =4950a(11) =9400a(12) =17900a(13) =34000a(14) =64750a(15) =123000a(16) =234250a(17) =445000a(18) =847500a(19) =1610000a(20) =3066250a(21) =5825000a(22) =11093750a(23) =21075000a(24) =40137500a(25) =76250000a(26) =145218750a(27) =275875000a(28) =525406250a(29) =998125000