1,2

a(n)>=A056841(n) since the trees of A056841 are a subset of these here. Edges along diagonals may cross.

M. Vicher, Polyforms

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.

See also A056840, A056841.

Sequence in context: A081096 A119692 A098854 this_sequence A098852 A120592 A081471

Adjacent sequences: A056784 A056785 A056786 this_sequence A056788 A056789 A056790

nonn

James A. Sellers (sellersj(AT)math.psu.edu), Aug 28 2000

Edited by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Apr 13 2006