Random walks.

Terms

a(0) =9a(1) =53a(2) =260a(3) =1156a(4) =4845a(5) =19551a(6) =76912a(7) =297275a(8) =1134705a(9) =4292145a(10) =16128061a(11) =60304951a(12) =224660626a(13) =834641671a(14) =3094322026a(15) =11453607152a(16) =42344301686a(17) =156404021389a(18) =577291806894