a(n) = (n+1)*(3*n-2)*C(2*n,n-1)/(4*n-2).

Terms

a(0) =1a(1) =8a(2) =42a(3) =200a(4) =910a(5) =4032a(6) =17556a(7) =75504a(8) =321750a(9) =1361360a(10) =5727436a(11) =23984688a(12) =100053772a(13) =416024000a(14) =1725013800a(15) =7135405920a(16) =29452939110a(17) =121347523440a(18) =499132441500