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

Terms

a(0) =1a(1) =2a(2) =4a(3) =15a(4) =64a(5) =317a(6) =1728a(7) =10268a(8) =65536a(9) =445376a(10) =3200000a(11) =24172677a(12) =191102976a(13) =1575167570a(14) =13492928512a(15) =119786923327