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%I #30 Nov 06 2016 08:56:48
%S 1,1,1,1,1,3,4
%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 if we form a linkage with centers of pennies as hinges and with struts between centers of two touching pennies, the linkage is rigid.
%C The pennies are laid flat on a horizontal plane. - _Daniel Forgues_, Oct 10 2016
%C 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
%e Examples for n=2,3,4,5,6,7:
%e n=2:
%e .o.o
%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 n=6:
%e .o.o.o
%e o.o.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
%e ..o.o
%Y Cf. A170807, A001524.
%K nonn,more
%O 1,6
%A _J. Lowell_, Dec 12 2009
%E Edited by _N. J. A. Sloane_, Dec 19 2009
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