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A077014
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Number of ways that a directed line (or river) that starts in the South can cross an East-West road n times.
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1
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2, 1, 2, 2, 6, 8, 28, 42, 162, 262, 1076, 1828, 7852, 13820, 61388, 110954, 505878, 933458, 4345660, 8152860, 38608380, 73424650, 352686780, 678390116, 3298016912, 6405031050, 31461151108, 61606881612, 305327366988, 602188541928, 3007925909860, 5969806669034
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,1
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COMMENTS
| The difference between this and A005316 is that here the river can also end up in the South-West. In A005316 it was required to end up in the North-East or South-East.
Or, number of ways that an undirected line that starts and ends anywhere can cross an East-West road n times.
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FORMULA
| Equals A005316(n) if n odd, or 2*A005316(n) if n even (because then each solution to A005316 can be reflected in a vertical mirror to give another solution).
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CROSSREFS
| Sequence in context: A054706 A081727 A000020 * A093655 A023140 A145859
Adjacent sequences: A077011 A077012 A077013 * A077015 A077016 A077017
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com) and Jon Wild (wild(AT)music.mcgill.ca), Nov 29 2002
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