a(n) is the number of imprimitive (periodic) 2n-bead balanced binary necklaces.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =2a(5) =1a(6) =5a(7) =1a(8) =10a(9) =4a(10) =27a(11) =1a(12) =88a(13) =1a(14) =247a(15) =29a(16) =810a(17) =1a(18) =2780a(19) =1a(20) =9260a(21) =249a(22) =32067a(23) =1a(24) =113520a(25) =26a(26) =400025a(27) =2704a(28) =1432868a(29) =1