A026872 T(2n,n+1), T given by A026758.
1, 4, 16, 65, 268, 1120, 4738, 20265, 87542, 381600, 1677170, 7427098, 33118073, 148618605, 670849720, 3044548675, 13886260129, 63627798355, 292789942333, 1352614829275, 6271530026012, 29176789293146, 136163014291526, 637294991967740
Offset: 1