a(n) = Sum_{k=0..n} binomial(n,k)*k!!*(n - k)!!.

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =64a(5) =230a(6) =936a(7) =3822a(8) =17344a(9) =78354a(10) =389280a(11) =1913010a(12) =10267776a(13) =54235350a(14) =311348352a(15) =1751907150a(16) =10673326080a(17) =63531238050a(18) =408231498240