OFFSET
1,1
COMMENTS
This sequence is based on the same Aztec diamond graph considered in A253107. In particular, it is the grid graph bounded by the eight equations x+y=-2n, x+y=2n, x-y=-2n, x-y=2n, x=1-2n, x=2n-1, y=1-2n, and y=2n-1.
An Eulerian orientation of a graph is an orientation of the edges such that every vertex has in-degree equal to out-degree.
All terms are even since reversing the orientation of every arc in any solution gives another solution.
EXAMPLE
a(2) = 18 because the edges of the graph illustrated below can be oriented in 18 different ways such that every vertex has in-degree equal to out-degree.
o---o
| |
o---o---o---o
| | | |
o---o---o---o
| |
o---o
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, May 22 2020
STATUS
approved