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

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =10a(5) =18a(6) =32a(7) =55a(8) =98a(9) =173a(10) =307a(11) =543a(12) =962a(13) =1705a(14) =3021a(15) =5354a(16) =9489a(17) =16818a(18) =29810a(19) =52836a(20) =93649a(21) =165987a(22) =294205a(23) =521464a(24) =924270a(25) =1638225a(26) =2903678a(27) =5146634a(28) =9122172a(29) =16168628