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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

Table of n, a(n) for n=0..21.

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 June 21 21:16 EDT 2018. Contains 305640 sequences. (Running on oeis4.)