a(n) = 2*(2n-1)!!-(n-1)!*2^(n-1), where (2n-1)!! is A001147(n).

A000779

a(n) = 2*(2n-1)!!-(n-1)!*2^(n-1), where (2n-1)!! is A001147(n).

Terms

    a(0) =1a(1) =4a(2) =22a(3) =162a(4) =1506a(5) =16950a(6) =224190a(7) =3408930a(8) =58596930a(9) =1123663590a(10) =23782729950a(11) =550718680050

External references