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 A077054 Number of ways a river can cross a road 2n times. 4
 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; text; internal format)
 OFFSET 0,3 COMMENTS More precisely, number of ways that a river (or directed line) that starts in the southwest 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. LINKS FORMULA a(n) = A005316(2*n). CROSSREFS Bisection of A005316. Cf. A005315, A085873, A086031. Sequence in context: A256338 A256331 A292875 * A205580 A032332 A032080 Adjacent sequences:  A077051 A077052 A077053 * A077055 A077056 A077057 KEYWORD nonn AUTHOR N. J. A. Sloane and Jon Wild, Nov 29 2002 STATUS approved

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Last modified August 17 23:07 EDT 2018. Contains 313817 sequences. (Running on oeis4.)