Number of (undirected) paths in the grid graph P_3 X P_n.

Terms

a(0) =3a(1) =49a(2) =322a(3) =1618a(4) =7119a(5) =28917a(6) =111360a(7) =412998a(8) =1490095a(9) =5266919a(10) =18329164a(11) =63032354a(12) =214788229a(13) =726753651a(14) =2445603148a(15) =8194841668a(16) =27369348241a(17) =91175639607a(18) =303133174298