a(n) = 6*(2n-4)!/((n-2)!n!), if n>2. a(0) = 1, a(1) = a(2) = 2.

Terms

a(0) =1a(1) =2a(2) =2a(3) =2a(4) =3a(5) =6a(6) =14a(7) =36a(8) =99a(9) =286a(10) =858a(11) =2652a(12) =8398a(13) =27132a(14) =89148a(15) =297160a(16) =1002915a(17) =3421710a(18) =11785890a(19) =40940460a(20) =143291610a(21) =504932340a(22) =1790214660a(23) =6382504440a(24) =22870640910a(25) =82334307276a(26) =297670187844