Number of n step walks (each step +-1 starting from 0) which are never more than 4 or less than -4.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =16a(5) =30a(6) =60a(7) =110a(8) =220a(9) =400a(10) =800a(11) =1450a(12) =2900a(13) =5250a(14) =10500a(15) =19000a(16) =38000a(17) =68750a(18) =137500a(19) =248750a(20) =497500a(21) =900000a(22) =1800000a(23) =3256250a(24) =6512500a(25) =11781250a(26) =23562500a(27) =42625000a(28) =85250000a(29) =154218750