a(n) = floor(n! / 4^n).

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =0a(9) =1a(10) =3a(11) =9a(12) =28a(13) =92a(14) =324a(15) =1217a(16) =4871a(17) =20703a(18) =93166a(19) =442542a(20) =2212711a(21) =11616735a(22) =63892044a(23) =367379253a(24) =2204275523a(25) =13776722023a(26) =89548693155a(27) =604453678799