Number of invertible circulant (0,1) matrices over the reals that have even determinant.

Terms

a(0) =0a(1) =0a(2) =3a(3) =0a(4) =15a(5) =12a(6) =77a(7) =32a(8) =261a(9) =260a(10) =1023a(11) =1056a(12) =4095a(13) =6552a(14) =19905a(15) =15872a(16) =66045a(17) =97740a(18) =262143a(19) =321320a(20) =1404375a(21) =1391720a(22) =4198397a(23) =6108912a(24) =17619525a(25) =23153832a(26) =79255071a(27) =116921224a(28) =268435455a(29) =529405320