a(n) = (A000110(n) - A000994(n+2))/2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =8a(6) =47a(7) =259a(8) =1437a(9) =8208a(10) =48659a(11) =300553a(12) =1937958a(13) =13052028a(14) =91784452a(15) =673328720a(16) =5146726651a(17) =40934788112a(18) =338292605257