0,4

The meandric numbers A005316 are the numbers of paths which cross themselves 0 times.

This sequence is the number of paths that must cross themselves exactly once.

Table of n, a(n) for n=0..11.

B. W. J. Irwin, On the number of k-crossing partitions

a(2) = 1, this is from the partition (2,1).

a(3) = 4, this is from the partitions (1,3,2),(2,1,3),(2,3,1),(3,1,2).

Cf. A005316, A287221.

Sequence in context: A222506 A108596 A204270 * A220205 A196880 A088013

Adjacent sequences: A287217 A287218 A287219 * A287221 A287222 A287223

nonn,more

Benedict W. J. Irwin, May 22 2017

approved