No closed formula is known.
Table of n, a(n) for n=1..10.
Z. C. Gao, V. A. Liskovets and N. C. Wormald, Enumeration of unrooted odd-valent regular planar maps, Preprint, 2005.
Cf. A112948, A112950.
Sequence in context: A160374 A127908 A015105 * A119621 A142734 A120773
Adjacent sequences: A112946 A112947 A112948 * A112950 A112951 A112952
Valery A. Liskovets, Oct 10 2005