Number of ways of placing n nonattacking range-3 leprechauns on an n X n board.

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =0a(9) =0a(10) =0a(11) =0a(12) =0a(13) =0a(14) =0a(15) =2a(16) =34a(17) =4a(18) =112a(19) =516a(20) =7312a(21) =81324a(22) =1056560a(23) =13443944a(24) =171919446a(25) =2195838076a(26) =28876216900a(27) =379982087060