A244522 Number of simple involutions of length n.

1, 1, 2, 0, 0, 2, 4, 10, 36, 126, 454, 1744, 6892, 28486, 120962, 531660, 2398248, 11127674, 52888998, 257570104, 1282439216

Offset

0,3

Links

Table of n, a(n) for n=0..20.

Example

The a(7) = 10 simple involutions are:
01:  [ 5 1 4 6 2 0 3 ]    (0, 5) (1) (2, 4) (3, 6)
02:  [ 4 1 6 3 0 5 2 ]    (0, 4) (1) (2, 6) (3) (5)
03:  [ 4 6 2 5 0 3 1 ]    (0, 4) (1, 6) (2) (3, 5)
04:  [ 5 3 6 1 4 0 2 ]    (0, 5) (1, 3) (2, 6) (4)
05:  [ 3 1 5 0 6 2 4 ]    (0, 3) (1) (2, 5) (4, 6)
06:  [ 3 5 2 0 6 1 4 ]    (0, 3) (1, 5) (2) (4, 6)
07:  [ 3 6 4 0 2 5 1 ]    (0, 3) (1, 6) (2, 4) (5)
08:  [ 2 5 0 3 6 1 4 ]    (0, 2) (1, 5) (3) (4, 6)
09:  [ 2 5 0 6 4 1 3 ]    (0, 2) (1, 5) (3, 6) (4)
10:  [ 2 4 0 6 1 5 3 ]    (0, 2) (1, 4) (3, 6) (5)

Crossrefs

Cf. A000085 (involutions), A111111 (simple permutations).

Sequence in context: A127862 A342223 A223142 * A292945 A024690 A200728

Adjacent sequences: A244519 A244520 A244521 * A244523 A244524 A244525

Keyword

nonn,more

Author

Joerg Arndt and Jay Pantone, Jul 21 2014

Status

approved