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A287221 Number of twice-crossing partitions on n nodes. 2

%I

%S 0,0,0,0,8,42,168,760,2418,10490,30842,131676

%N Number of twice-crossing partitions on n nodes.

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

%C This sequence is the number of paths that must cross themselves exactly twice.

%H B. W. J. Irwin, <a href="https://www.authorea.com/users/5445/articles/173040-on-the-number-of-k-crossing-partitions/_show_article">On the number of k-crossing partitions</a>

%e a(4) = 8; this is from the partitions

%e (2,1,4,3), (2,4,3,1),

%e (3,2,4,1), (3,4,2,1),

%e (4,1,3,2), (4,2,1,3),

%e (4,2,3,1), (4,3,1,2).

%Y Cf. A005316, A287220.

%K nonn,more

%O 0,5

%A _Benedict W. J. Irwin_, May 22 2017

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Last modified December 3 13:27 EST 2021. Contains 349463 sequences. (Running on oeis4.)