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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
1, 0, 1, 1, 2, 4, 7
OFFSET
1,5
EXAMPLE
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.
Sequence in context: A218087 A090315 A083753 * A221840 A111512 A283509
KEYWORD
nonn,more
AUTHOR
N. J. A. Sloane, Dec 17 2009
EXTENSIONS
a(6) and a(7) corrected by John W. Layman, Dec 17 2009
STATUS
approved