a(n) = floor(sqrt((2*n)^n)).

Terms

a(0) =1a(1) =1a(2) =4a(3) =14a(4) =64a(5) =316a(6) =1728a(7) =10267a(8) =65536a(9) =445375a(10) =3200000a(11) =24172676a(12) =191102976a(13) =1575167569a(14) =13492928512a(15) =119786923326