a(n) = floor( exp(19/21)*n! ).

Terms

a(0) =2a(1) =4a(2) =14a(3) =59a(4) =296a(5) =1779a(6) =12455a(7) =99644a(8) =896801a(9) =8968011a(10) =98648121a(11) =1183777460a(12) =15389106991a(13) =215447497884