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%I #40 Aug 17 2020 07:32:01
%S 1,38,675,7840,74291,630546,5014843,38290580,284670771,2078666854,
%T 14991726971,107192696520,761785678291,5390429149562,38025671502459,
%U 267655258816060,1881013969942451,13204432684440270
%N Number of proper (n,3)-clusterings.
%C See Roman et al. for precise definition.
%H Adam Roman, Igor T. Podolak and Agnieszka Deszynska, <a href="https://www.ejournals.eu/sj/index.php/SI/article/view/2227">On the number of clusterings in a hierarchical classification model with overlapping clusters</a>, Schedae Informaticae, Volume 20, 2011.
%F Roman et al. give an explicit formula.
%o (PARI) a(n) = {if (n==3, return (1)); n --; return (7*a(n) + 2*5^n - 3*4^n + (4*n-11)*3^n/6 + (4 - 15*n/4)*2^n + 9*n - 1/2);} \\ _Michel Marcus_, Apr 21 2013
%K nonn
%O 3,2
%A _N. J. A. Sloane_, May 06 2012
%E More terms from _Michel Marcus_, Apr 21 2013
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