a(n) = floor( exp(5/7)*n! ).

Terms

a(0) =2a(1) =4a(2) =12a(3) =49a(4) =245a(5) =1470a(6) =10295a(7) =82362a(8) =741264a(9) =7412647a(10) =81539127a(11) =978469535a(12) =12720103955a(13) =178081455373