OFFSET
4,1
COMMENTS
A meander together with the horizontal line separates the plane into several connected components. Each component has a given number of edges which is always an even number. The digons (or bigons) are the faces with least number of edges, that is 2. Equivalently, the number of digons is the number of arches between adjacent sites ("minimal arches") where the two extremal ones are considered adjacent.
LINKS
Vincent Delecroix, Table of n, a(n) for n = 4..164
V. Delecroix, E. Goujard, P. Zograf, A. Zorich Enumeration of meanders and Masur-Veech volumes, arXiv:1705.05190 [math.GT], 2017.
FORMULA
Known asymptotics: Sum_{n <= N} a(n) ~ 16 N^5/(3 Pi^4).
CROSSREFS
KEYWORD
nonn
AUTHOR
Vincent Delecroix, Mar 14 2018
STATUS
approved