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%I #14 Oct 29 2013 11:22:06
%S 1,2,4,10,24,62,154,396,992,2536,6376,16238,40914,103954,262298,
%T 665478,1680726,4260262,10766470,27274444
%N Number of involutions avoiding the pattern 2431.
%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 26, only 35142 and 52431 contain the pattern 2431, so a(5) = 24.
%Y Cf. A001006, A121704, A230552-A230556.
%K nonn
%O 1,2
%A _Cheyne Homberger_, Oct 23 2013
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