A045628 Number of 2n-bead black-white reversible necklaces with n black beads and fundamental period 2n.

1, 1, 1, 2, 6, 15, 46, 132, 432, 1384, 4735, 16158, 56766, 200473, 718012, 2587000, 9398080, 34324173, 126067124, 465093570, 1723171550, 6407924165, 23910560070, 89494164972, 335913861648, 1264107416450, 4768452532025