A077054 - OEIS

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A077054

Number of ways a river can cross a road 2n times.

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 November 11 23:22 EST 2017. Contains 294529 sequences.