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

Author

Robert Price, May 07 2012

Keywords

• nonn

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.

Extensions

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