OFFSET
3,2
COMMENTS
See Roman et al. for precise definition.
LINKS
Adam Roman, Igor T. Podolak and Agnieszka Deszynska, On the number of clusterings in a hierarchical classification model with overlapping clusters, Schedae Informaticae, Volume 20, 2011.
FORMULA
Roman et al. give an explicit formula.
PROG
(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
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, May 06 2012
EXTENSIONS
More terms from Michel Marcus, Apr 21 2013
STATUS
approved