a(n) = binomial(n,17).

Terms

a(0) =1a(1) =18a(2) =171a(3) =1140a(4) =5985a(5) =26334a(6) =100947a(7) =346104a(8) =1081575a(9) =3124550a(10) =8436285a(11) =21474180a(12) =51895935a(13) =119759850a(14) =265182525a(15) =565722720a(16) =1166803110a(17) =2333606220a(18) =4537567650a(19) =8597496600a(20) =15905368710a(21) =28781143380a(22) =51021117810a(23) =88732378800a(24) =151584480450a(25) =254661927156