Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #15 Oct 24 2021 04:33:49
%S 1,1,2,4,9,21,51,128,327,858,2272,6146,16716,46246,128414,361493,
%T 1020506,2913060,8335405,24067930,69646035
%N Number of involutions avoiding the pattern 4231.
%H Miklós Bóna, Cheyne Homberger, Jay Pantone, and Vince Vatter, <a href="http://arxiv.org/abs/1310.7003">Pattern-avoiding involutions: exact and asymptotic enumeration</a>, arxiv:1310.7003, 2013.
%e Of the 26 involutions of length 5, only 53241, 42315, 52431, 52341, and 15342 contain the pattern 4231, so a(5) = 21.
%Y Cf. A000085, A001006, A121704, A230551-A230555.
%K nonn,more
%O 0,3
%A _Cheyne Homberger_, Oct 23 2013