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%I #14 Oct 28 2013 13:30:20
%S 1,1,2,4,9,21,51,126,321,820,2160,5654,15272,40758,112280,304471,
%T 852164,2341980,6640755,18460066,52915999
%N Number of involutions avoiding the pattern 1324.
%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 21435, 13254, 13245, 14325, and 12435 contain the pattern 1324, so a(5) = 21.
%Y Cf. A000085, A001006, A121704, A230551-A230556.
%K nonn
%O 0,3
%A _Cheyne Homberger_, Oct 23 2013
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