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%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
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