A204352 Meandric numbers for a river crossing up to 3 parallel roads at n points.

1, 1, 2, 4, 9, 21, 52, 131, 345, 915, 2519, 6926, 19711, 55674, 162594, 468929, 1398129, 4100003, 12433282, 36960316, 113678461, 341785050, 1063890616, 3229522688, 10156518859, 31085477306, 98635931623, 304048850048, 972323924567, 3015979607106, 9711570455824

Offset

0,3

Comments

Number of ways that a river (or directed line) that starts in the South and flows East can cross 3 parallel East-West roads n times.

Sequence derived from list of solutions described in A206432.

Links

Andrew Howroyd, Table of n, a(n) for n = 0..40

Crossrefs

Column k=3 of A380367.

Cf. A005316 (sequence for one road; extensive references and links).

Cf. A076876 (sequence for two parallel roads).

Cf. A204352, A208062, A208126, A208452, A208453, A209383, A209621, A209622, A209626, A209656, A209657, A209660, A209707, A210344, A210478, A210567, A210592 (sequences for 3 to 19 parallel roads).

Cf. A206432 (sequence for unlimited number of parallel roads).

Cf. A076875, A076906, A076907.

Sequence in context: A333105 A148071 A000636 * A195980 A136753 A289666

Adjacent sequences: A204349 A204350 A204351 * A204353 A204354 A204355

Keyword

nonn

Author

Robert Price, May 07 2012

Extensions

a(21)-a(40) from Andrew Howroyd, Dec 07 2015

Status

approved