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 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. 2
 1, 1, 1, 1, 1, 3, 4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 COMMENTS The pennies are laid flat on a horizontal plane. - Daniel Forgues, Oct 10 2016 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 LINKS Table of n, a(n) for n=1..7. EXAMPLE Examples for n=2,3,4,5,6,7: n=2: .o.o n=3: ..o .o.o n=4: ..o .o.o ..o n=5: ..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 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 CROSSREFS Cf. A170807, A001524. Sequence in context: A081174 A292832 A264932 * A132139 A045951 A238797 Adjacent sequences: A171601 A171602 A171603 * A171605 A171606 A171607 KEYWORD nonn,more AUTHOR J. Lowell, Dec 12 2009 EXTENSIONS Edited by N. J. A. Sloane, Dec 19 2009 STATUS approved

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Last modified February 29 20:48 EST 2024. Contains 370428 sequences. (Running on oeis4.)