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A171604
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Take the standard 2-D lattice packing of pennies; a(n) = number of ways to pick n pennies (modulo rotations and reflections) such that if we form a linkage with centers of pennies as hinges and with struts between centers of two touching pennies, the linkage is rigid.
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2
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OFFSET
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1,6
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COMMENTS
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The pennies are laid flat on a horizontal plane. - Daniel Forgues, Oct 10 2016
We might have a rigid structure with a hole through which we have a taut chain of pennies (is this considered a packing?). - Daniel Forgues, Oct 08 2016
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LINKS
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EXAMPLE
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Examples for n=2,3,4,5,6,7:
n=2:
.o.o
n=3:
..o
.o.o
n=4:
..o
.o.o
..o
n=5:
..o.o
.o.o.o
n=6:
.o.o.o
o.o.o
.
...o
o.o.o
.o.o
.
..o
.o.o
o.o.o
n=7:
..o.o.o
.o.o.o.o
.
..o.o
.o.o.o
..o.o
.
...o.o
..o.o
.o.o.o
.
....o.o
...o.o.o
..o.o
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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