a(0)=0, a(1)=0; for n>1, a(n) = a(n-1) + (n-3)*a(n-2) + 1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =4a(5) =9a(6) =22a(7) =59a(8) =170a(9) =525a(10) =1716a(11) =5917a(12) =21362a(13) =80533a(14) =315516a(15) =1281913a(16) =5383622a(17) =23330405a(18) =104084736a(19) =477371217a(20) =2246811730a(21) =10839493637a(22) =53528916508a(23) =270318789249