a(n) = floor(sqrt((2*n)^(n+1))).

Terms

a(0) =0a(1) =2a(2) =8a(3) =36a(4) =181a(5) =1000a(6) =5985a(7) =38416a(8) =262144a(9) =1889568a(10) =14310835a(11) =113379904a(12) =936209559a(13) =8031810176a(14) =71397866601a(15) =656100000000