A230555 Number of involutions avoiding 3421.

1, 1, 2, 4, 10, 25, 66, 173, 460, 1218, 3240, 8602, 22878, 60794, 161668, 429752, 1142758, 3038173, 8078606, 21479469, 57113888

Offset: 0

Author

Cheyne Homberger, Oct 23 2013

Keywords

• nonn

Examples

			Of the 26 involutions of length 5, only 45312 contains the pattern 3421, so a(5) = 25.
		

Links

• Miklós Bóna, Cheyne Homberger, Jay Pantone, and Vince Vatter, Pattern-avoiding involutions: exact and asymptotic enumeration, arxiv:1310.7003, 2013.

Crossrefs

Cf. A000085, A001006, A121704, A230551-A230556.