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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

Table of n, a(n) for n=1..7.

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

Cf. A170807, A001524.

Sequence in context: A076253 A081174 A264932 * A132139 A045951 A238797

Adjacent sequences:  A171601 A171602 A171603 * A171605 A171606 A171607

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 September 25 12:56 EDT 2017. Contains 292469 sequences.