Number of (1,1)-steps in all weighted lattice paths in L_n.

Terms

a(0) =0a(1) =0a(2) =0a(3) =2a(4) =6a(5) =18a(6) =56a(7) =162a(8) =462a(9) =1306a(10) =3648a(11) =10116a(12) =27892a(13) =76524a(14) =209112a(15) =569506a(16) =1546542a(17) =4189314a(18) =11323480a(19) =30548190a(20) =82272330a(21) =221240070a(22) =594131160a(23) =1593553452a(24) =4269391596a(25) =11426761548a(26) =30554523096a(27) =81631135502a(28) =217918012002