

A064640


Positions of noncrossing fixedpointfree involutions (encoded by A014486) in A055089, sorted to ascending order.


8



0, 1, 7, 23, 127, 143, 415, 659, 719, 5167, 5183, 5455, 5699, 5759, 16687, 16703, 26815, 28495, 36899, 36959, 38579, 40031, 40319, 368047, 368063, 368335, 368579, 368639, 379567, 379583, 389695, 391375, 399779, 399839, 401459, 402911, 403199
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OFFSET

0,3


COMMENTS

These permutations belong to the interpretation (kk) of the exercise 19 in the sixth chapter "Exercises on Catalan and Related Numbers" of Enumerative Combinatorics, Vol. 2, 1999 by R. P. Stanley, Wadsworth, Vol. 1, 1986: Fixedpointfree involutions w of [2n] such that if i < j < k < l and w(i) = k, then w(j) <> l.
From this, it follows that when they are subjected to the same automorphism as used in A061417 and A064636, one gets A002995.


LINKS



EXAMPLE

The first eight such permutations (after the identity) are in positions 1, 7, 23, 127, 143, 415, 659, 719 of A055089: 21, 2143, 4321, 214365, 432165, 216543, 632541, 654321 which written as disjoint cycles are (1 2), (1 2)(3 4), (1 4)(2 3), (1 2)(3 4)(5 6), (1 4)(2 3)(5 6), (1 2)(3 6)(4 5), (1 6)(2 3)(4 5), (1 6)(2 5)(3 4).


MAPLE



CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



