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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

%I #6 Sep 29 2013 03:36:12

%S 1,0,1,1,2,4,7

%N 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.

%e Examples for n=3,4,5,6,7:

%e n=3:

%e ..o

%e .o.o

%e n=4:

%e ..o

%e .o.o

%e ..o

%e n=5:

%e ..o.o

%e .o.o.o

%e .

%e ....o

%e .o.o.o

%e ..o

%e n=6:

%e .o.o.o

%e o.o.o

%e .

%e ...o.o

%e o.o.o

%e .o

%e .

%e ...o

%e o.o.o

%e .o.o

%e .

%e ..o

%e .o.o

%e o.o.o

%e n=7:

%e ..o.o.o

%e .o.o.o.o

%e .

%e ..o.o

%e .o.o.o

%e ..o.o

%e .

%e ...o.o

%e ..o.o

%e .o.o.o

%e .

%e ....o.o

%e .o.o.o.o

%e ..o

%e .

%e ....o.o

%e ...o.o.o

%e ..o.o

%e .

%e ....o

%e .o.o.o.o

%e ..o...o

%e .

%e .....o.o

%e ..o.o.o

%e .o.o

%Y Cf. A171604.

%K nonn,more

%O 1,5

%A _N. J. A. Sloane_, Dec 17 2009

%E a(6) and a(7) corrected by _John W. Layman_, Dec 17 2009

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Last modified February 26 05:55 EST 2024. Contains 370335 sequences. (Running on oeis4.)