Number of n-step random walks on a line starting from the origin and returning to it at least once.

Terms

a(0) =0a(1) =0a(2) =2a(3) =4a(4) =10a(5) =20a(6) =44a(7) =88a(8) =186a(9) =372a(10) =772a(11) =1544a(12) =3172a(13) =6344a(14) =12952a(15) =25904a(16) =52666a(17) =105332a(18) =213524a(19) =427048a(20) =863820a(21) =1727640a(22) =3488872a(23) =6977744a(24) =14073060a(25) =28146120a(26) =56708264a(27) =113416528a(28) =228318856a(29) =456637712