a(n) = ceiling(e^(n*(Pi/2))).

Terms

a(0) =1a(1) =5a(2) =24a(3) =112a(4) =536a(5) =2576a(6) =12392a(7) =59610a(8) =286752a(9) =1379411a(10) =6635624a(11) =31920520a(12) =153552936a(13) =738662923a(14) =3553321281a(15) =17093171649a(16) =82226315586a(17) =395547831245