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EXAMPLE
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For n=2 we have
o-o
and
..o
./.
o..
as the only a(2)=2 candidates. Trees contributing to n=7 are
o.o-o
|\.\.
o.o-o
...\.
....o
or
o....
|\...
o.o-o
...X.
..o.o
./...
o....
where dashes are edges in E, NE, N, NW, W, SW, S or SE direction
that connect nodes marked 'o' horizontally, vertically or along diagonals,
and X's are crossing diagonal edges.
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