Number of ways to place 3 nonattacking zebras on a 3 X n board.

Terms

a(0) =1a(1) =20a(2) =84a(3) =200a(4) =403a(5) =720a(6) =1180a(7) =1808a(8) =2631a(9) =3676a(10) =4970a(11) =6540a(12) =8413a(13) =10616a(14) =13176a(15) =16120a(16) =19475a(17) =23268a(18) =27526a(19) =32276a(20) =37545a(21) =43360a(22) =49748a(23) =56736a(24) =64351a(25) =72620a(26) =81570a(27) =91228a(28) =101621a(29) =112776