OFFSET
1,2
COMMENTS
Previous name was: Number of link shadows with n crossings.
A link shadow is a connected plane 4-regular multigraph, allowing loops and multiple edges. Also: number of plane quadrangulations (allowing multiple edges) with n+2 vertices. In each case the sequence counts isomorphism classes which respect the embedding, with mirror image being an isomorphism. - Brendan McKay, Mar 10 2023
LINKS
J. Cantarella, H. Chapman, and M. Mastin, Knot Probabilities in Random Diagrams, arXiv preprint arXiv:1512.05749 [math.GT], 2015. See Tables 1.
Richard Kapolnai, Gabor Domokos, and Timea Szabo, Generating spherical multiquadrangulations by restricted vertex splittings and the reducibility of equilibrium classes, Periodica Polytechnica Electrical Engineering, 56(1):11-10, 2012. Also arXiv:1206.1698, 2012.
EXAMPLE
a(1)=1 corresponds to a single vertex with two loops. a(2)=3 corresponds to two vertices joined by 4 edges or two vertices with loops joined by 2 edges. In the last case, the two loops may lie in the same face or different faces and these are not isomorphic.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
N. J. A. Sloane, Nov 07 2016
EXTENSIONS
a(11)-a(18) from Heidi Van den Camp and Brendan McKay, Mar 10 2023
STATUS
approved