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A077054
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Number of ways a river can cross a road 2n times.
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4
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1, 1, 3, 14, 81, 538, 3926, 30694, 252939, 2172830, 19304190, 176343390, 1649008456, 15730575554, 152663683494, 1503962954930, 15012865733351, 151622652413194, 1547365078534578, 15939972379349178, 165597452660771610, 1733609081727968492
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| More precisely, number of ways that a river (or directed line) that starts in the South-West and flows East can cross an East-West road 2n times (bisection of A005316).
Also number of ways a loop can cross two parallel roads 2n times. Some portion of loop must lie below lower road.
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CROSSREFS
| Bisection of A005316. Cf. A005315, A085873, A086031.
Sequence in context: A000264 A009053 A202474 * A205580 A032332 A032080
Adjacent sequences: A077051 A077052 A077053 * A077055 A077056 A077057
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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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