a(n) = floor(exp(n*Pi/2)).

Terms

a(0) =1a(1) =4a(2) =23a(3) =111a(4) =535a(5) =2575a(6) =12391a(7) =59609a(8) =286751a(9) =1379410a(10) =6635623a(11) =31920519a(12) =153552935a(13) =738662922a(14) =3553321280a(15) =17093171648a(16) =82226315585a(17) =395547831244