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%I #12 Oct 28 2013 13:30:31
%S 1,1,2,4,10,25,66,173,460,1218,3240,8602,22878,60794,161668,429752,
%T 1142758,3038173,8078606,21479469,57113888
%N Number of involutions avoiding 3421.
%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 45312 contains the pattern 3421, so a(5) = 25.
%Y Cf. A000085, A001006, A121704, A230551-A230556.
%K nonn
%O 0,3
%A _Cheyne Homberger_, Oct 23 2013
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