Number of ways to arrange 4 nonattacking knights on the lower triangle of an n X n board.

Terms

a(0) =0a(1) =0a(2) =4a(3) =46a(4) =407a(5) =2168a(6) =8685a(7) =28376a(8) =79611a(9) =198334a(10) =449336a(11) =942072a(12) =1852096a(13) =3449261a(14) =6133944a(15) =10482661a(16) =17304542a(17) =27710241a(18) =43194961a(19) =65737379a(20) =97916361a(21) =143047462a(22) =205341311a(23) =290086086a(24) =403856389a(25) =554750936a(26) =752661582a(27) =1009576306a(28) =1339918886