Number of n-step walks (each step +-1 starting from 0) which are never more than 2 or less than -2.

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =12a(5) =18a(6) =36a(7) =54a(8) =108a(9) =162a(10) =324a(11) =486a(12) =972a(13) =1458a(14) =2916a(15) =4374a(16) =8748a(17) =13122a(18) =26244a(19) =39366a(20) =78732a(21) =118098a(22) =236196a(23) =354294a(24) =708588a(25) =1062882a(26) =2125764a(27) =3188646a(28) =6377292a(29) =9565938