cp's OEIS Frontend

A171604 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.

%I A171604 #30 Nov 06 2016 08:56:48
%S A171604 1,1,1,1,1,3,4
%N A171604 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 A171604 The pennies are laid flat on a horizontal plane. - _Daniel Forgues_, Oct 10 2016
%C A171604 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 A171604 Examples for n=2,3,4,5,6,7:
%e A171604 n=2:
%e A171604 .o.o
%e A171604 n=3:
%e A171604 ..o
%e A171604 .o.o
%e A171604 n=4:
%e A171604 ..o
%e A171604 .o.o
%e A171604 ..o
%e A171604 n=5:
%e A171604 ..o.o
%e A171604 .o.o.o
%e A171604 n=6:
%e A171604 .o.o.o
%e A171604 o.o.o
%e A171604 .
%e A171604 ...o
%e A171604 o.o.o
%e A171604 .o.o
%e A171604 .
%e A171604 ..o
%e A171604 .o.o
%e A171604 o.o.o
%e A171604 n=7:
%e A171604 ..o.o.o
%e A171604 .o.o.o.o
%e A171604 .
%e A171604 ..o.o
%e A171604 .o.o.o
%e A171604 ..o.o
%e A171604 .
%e A171604 ...o.o
%e A171604 ..o.o
%e A171604 .o.o.o
%e A171604 .
%e A171604 ....o.o
%e A171604 ...o.o.o
%e A171604 ..o.o
%Y A171604 Cf. A170807, A001524.
%K A171604 nonn,more
%O A171604 1,6
%A A171604 _J. Lowell_, Dec 12 2009
%E A171604 Edited by _N. J. A. Sloane_, Dec 19 2009