a(n) = T(2n,n-1), where T is the array in A026300.

Terms

a(0) =1a(1) =4a(2) =20a(3) =104a(4) =560a(5) =3080a(6) =17199a(7) =97120a(8) =553146a(9) =3171860a(10) =18288127a(11) =105922544a(12) =615820400a(13) =3591858480a(14) =21008159700a(15) =123169357728a(16) =723659556026