a(n) = A068875(n-1) - A003239(n).

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =2a(5) =4a(6) =18a(7) =48a(8) =156a(9) =472a(10) =1526a(11) =4852a(12) =16000a(13) =52940a(14) =178276a(15) =605520a(16) =2079862a(17) =7201084a(18) =25138878a(19) =88358520a(20) =312576996a(21) =1112087012a(22) =3977502766a(23) =14294093652a(24) =51596165872a(25) =186997738504a(26) =680272334202