Number of ways to place 4 nonattacking semi-queens on an n X n board.

Terms

a(0) =0a(1) =0a(2) =0a(3) =7a(4) =223a(5) =2429a(6) =15045a(7) =66122a(8) =230074a(9) =675798a(10) =1745318a(11) =4073993a(12) =8764753a(13) =17630795a(14) =33522531a(15) =60756612a(16) =105666148a(17) =177293340a(18) =288246972a(19) =455749371a(20) =702898611a(21) =1060173961a(22) =1567213681a(23) =2274896558a(24) =3247759614a(25) =4566786770a(26) =6332604226a(27) =8669120733a(28) =11727651845