A170807 Take the standard 2-D lattice packing of pennies; a(n) = number of ways to pick n pennies (modulo rotations and reflections) such that the graph with nodes = centers of pennies, edges = pairs of touching pennies is connected and every edge belongs to at least one triangle.

1, 0, 1, 1, 2, 4, 7

Offset: 1

Author

N. J. A. Sloane, Dec 17 2009

Keywords

• nonn
• more

Examples

			Examples for n=3,4,5,6,7:
n=3:
..o
.o.o
n=4:
..o
.o.o
..o
n=5:
..o.o
.o.o.o
.
....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
.
..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
.
....o.o
...o.o.o
..o.o
.
....o
.o.o.o.o
..o...o
.
.....o.o
..o.o.o
.o.o
		

Crossrefs

Cf. A171604.

Extensions

a(6) and a(7) corrected by John W. Layman, Dec 17 2009