A230556 Number of involutions avoiding the pattern 4231.
1, 1, 2, 4, 9, 21, 51, 128, 327, 858, 2272, 6146, 16716, 46246, 128414, 361493, 1020506, 2913060, 8335405, 24067930, 69646035
Offset: 0
Examples
Of the 26 involutions of length 5, only 53241, 42315, 52431, 52341, and 15342 contain the pattern 4231, so a(5) = 21.
Links
- Miklós Bóna, Cheyne Homberger, Jay Pantone, and Vince Vatter, Pattern-avoiding involutions: exact and asymptotic enumeration, arxiv:1310.7003, 2013.