[seqfan] Re: a question about arrangements of pennies
William Keith
wjk26 at drexel.edu
Mon Dec 14 05:56:58 CET 2009
> There are several ways to enforce rigidity. Perhaps
> the best is to add a further constraint to the first
> problem. Form a second graph with nodes = triangles,
> edges = two triangles that share an edge. Then this second graph
> must be connected.
Counterexample!
****
*****
** **
* *
** **
*****
****
If that doesn't come across well graphically, picture a "mostly
hexagon" with 4 in the first row, 5 in the second, the third row being
2-blank-2, the fourth row being two pennies, just one in the middle of
each pair, and the bottom half a vertical reflection.
This is mechanically rigid -- I think that's the intuition you sought
to capture -- but the associated graph as you describe would be two
disconnected arcs.
(Which is not to say that your sequence isn't also interesting; it
would be counting such graphs with a weight of 3 per node, minus 2 per
edge, plus a somewhat more complicated summand accounting for the
possibility of cycles and part-cycles of various sizes. It would be a
"stronger" subset of rigid.)
William Keith
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