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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
 1, 0, 1, 1, 2, 4, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 LINKS Table of n, a(n) for n=1..7. EXAMPLE Examples for n=3,4,5,6,7: n=3: ..o .o.o n=4: ..o .o.o ..o n=5: ..o.o .o.o.o . ....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 . ..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 . ....o.o ...o.o.o ..o.o . ....o .o.o.o.o ..o...o . .....o.o ..o.o.o .o.o CROSSREFS Cf. A171604. Sequence in context: A218087 A090315 A083753 * A221840 A111512 A283509 Adjacent sequences: A170804 A170805 A170806 * A170808 A170809 A170810 KEYWORD nonn,more AUTHOR N. J. A. Sloane, Dec 17 2009 EXTENSIONS a(6) and a(7) corrected by John W. Layman, Dec 17 2009 STATUS approved

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Last modified November 30 19:18 EST 2023. Contains 367462 sequences. (Running on oeis4.)