Number of inequivalent (mod D_4) ways four checkers can be placed on an n X n board.

Terms

a(0) =1a(1) =23a(2) =252a(3) =1666a(4) =7509a(5) =26865a(6) =79920a(7) =209096a(8) =491425a(9) =1064575a(10) =2150076a(11) =4104738a(12) =7458437a(13) =13005041a(14) =21857984a(15) =35598880a(16) =56353185a(17) =87019191a(18) =131364700a(19) =194364050a(20) =282314901a(21) =403316353a(22) =567402672a(23) =787201416a(24) =1078078209a(25) =1459020095a(26) =1952782300a(27) =2587048786a(28) =3394568325