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.

1, 1, 1, 1, 1, 3, 4

Offset: 1

Author

J. Lowell, Dec 12 2009

Keywords

• nonn
• more

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

Examples

			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.

Extensions

Edited by N. J. A. Sloane, Dec 19 2009