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A171604 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. 2
1, 1, 1, 1, 1, 3, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,6
COMMENTS
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
LINKS
EXAMPLE
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
CROSSREFS
Sequence in context: A081174 A292832 A264932 * A132139 A045951 A238797
KEYWORD
nonn,more
AUTHOR
J. Lowell, Dec 12 2009
EXTENSIONS
Edited by N. J. A. Sloane, Dec 19 2009
STATUS
approved

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Last modified February 29 20:48 EST 2024. Contains 370428 sequences. (Running on oeis4.)