a(n) = Floor[(2*Pi/E)*n^2].

Terms

a(0) =2a(1) =9a(2) =20a(3) =36a(4) =57a(5) =83a(6) =113a(7) =147a(8) =187a(9) =231a(10) =279a(11) =332a(12) =390a(13) =453a(14) =520a(15) =591a(16) =668a(17) =748a(18) =834a(19) =924a(20) =1019a(21) =1118a(22) =1222a(23) =1331a(24) =1444a(25) =1562a(26) =1685a(27) =1812a(28) =1943a(29) =2080