Number of n-step walks on square lattice.

Terms

a(0) =2a(1) =6a(2) =18a(3) =50a(4) =142a(5) =390a(6) =1086a(7) =2958a(8) =8134a(9) =22050a(10) =60146a(11) =162466a(12) =440750a(13) =1187222a(14) =3208298a(15) =8622666a(16) =23233338a(17) =62329366a(18) =167558310a(19) =448848582a(20) =1204403014a(21) =3222280242a(22) =8633306906a(23) =23073198658a(24) =61740677454a(25) =164856393110a(26) =440658745814