Number of ways to arrange 2 nonattacking knights on the lower triangle of an n X n board.

Terms

a(0) =0a(1) =3a(2) =13a(3) =37a(4) =87a(5) =178a(6) =328a(7) =558a(8) =892a(9) =1357a(10) =1983a(11) =2803a(12) =3853a(13) =5172a(14) =6802a(15) =8788a(16) =11178a(17) =14023a(18) =17377a(19) =21297a(20) =25843a(21) =31078a(22) =37068a(23) =43882a(24) =51592a(25) =60273a(26) =70003a(27) =80863a(28) =92937a(29) =106312