cp's OEIS Frontend

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.

%I A170807 #6 Sep 29 2013 03:36:12
%S A170807 1,0,1,1,2,4,7
%N 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.
%e A170807 Examples for n=3,4,5,6,7:
%e A170807 n=3:
%e A170807 ..o
%e A170807 .o.o
%e A170807 n=4:
%e A170807 ..o
%e A170807 .o.o
%e A170807 ..o
%e A170807 n=5:
%e A170807 ..o.o
%e A170807 .o.o.o
%e A170807 .
%e A170807 ....o
%e A170807 .o.o.o
%e A170807 ..o
%e A170807 n=6:
%e A170807 .o.o.o
%e A170807 o.o.o
%e A170807 .
%e A170807 ...o.o
%e A170807 o.o.o
%e A170807 .o
%e A170807 .
%e A170807 ...o
%e A170807 o.o.o
%e A170807 .o.o
%e A170807 .
%e A170807 ..o
%e A170807 .o.o
%e A170807 o.o.o
%e A170807 n=7:
%e A170807 ..o.o.o
%e A170807 .o.o.o.o
%e A170807 .
%e A170807 ..o.o
%e A170807 .o.o.o
%e A170807 ..o.o
%e A170807 .
%e A170807 ...o.o
%e A170807 ..o.o
%e A170807 .o.o.o
%e A170807 .
%e A170807 ....o.o
%e A170807 .o.o.o.o
%e A170807 ..o
%e A170807 .
%e A170807 ....o.o
%e A170807 ...o.o.o
%e A170807 ..o.o
%e A170807 .
%e A170807 ....o
%e A170807 .o.o.o.o
%e A170807 ..o...o
%e A170807 .
%e A170807 .....o.o
%e A170807 ..o.o.o
%e A170807 .o.o
%Y A170807 Cf. A171604.
%K A170807 nonn,more
%O A170807 1,5
%A A170807 _N. J. A. Sloane_, Dec 17 2009
%E A170807 a(6) and a(7) corrected by _John W. Layman_, Dec 17 2009