

A171604


Take the standard 2D 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




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



