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A006602
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a(n) is the number of hierarchical models on n unlabeled factors or variables with linear terms forced.
(Formerly M1532)
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28
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OFFSET
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0,1
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COMMENTS
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Also number of pure (= irreducible) group-testing histories of n items - A. Boneh, Mar 31 2000
Also number of antichain covers of an unlabeled n-set, so a(n) equals first differences of A003182. - Vladeta Jovovic, Goran Kilibarda, Aug 18 2000
Also number of inequivalent (under permutation of variables) nondegenerate monotone Boolean functions of n variables. We say h and g (functions of n variables) are equivalent if there exists a permutation p of S_n such that hp=g. E.g., a(3)=5 because xyz, xy+xz+yz, x+yz+xyz, xy+xz+xyz, x+y+z+xy+xz+yz+xyz are 5 inequivalent nondegenerate monotone Boolean functions that generate (by permutation of variables) the other 4. For example, y+xz+xyz can be obtained from x+yz+xyz by exchanging x and y. - Alan Veliz-Cuba (alanavc(AT)vt.edu), Jun 16 2006
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REFERENCES
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Y. M. M. Bishop, S. E. Fienberg and P. W. Holland, Discrete Multivariate Analysis. MIT Press, 1975, p. 34. [In part (e), the Hierarchy Principle for log-linear models is defined. It essentially says that if a higher-order parameter term is included in the log-linear model, then all the lower-order parameter terms should also be included. - Petros Hadjicostas, Apr 10 2020]
V. Jovovic and G. Kilibarda, On enumeration of the class of all monotone Boolean functions, in preparation.
A. A. Mcintosh, personal communication.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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EXAMPLE
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Non-isomorphic representatives of the a(0) = 2 through a(4) = 20 antichains:
{} {{1}} {{12}} {{123}} {{1234}}
{{}} {{1}{2}} {{1}{23}} {{1}{234}}
{{13}{23}} {{12}{34}}
{{1}{2}{3}} {{14}{234}}
{{12}{13}{23}} {{1}{2}{34}}
{{134}{234}}
{{1}{24}{34}}
{{1}{2}{3}{4}}
{{13}{24}{34}}
{{14}{24}{34}}
{{13}{14}{234}}
{{12}{134}{234}}
{{1}{23}{24}{34}}
{{124}{134}{234}}
{{12}{13}{24}{34}}
{{14}{23}{24}{34}}
{{12}{13}{14}{234}}
{{123}{124}{134}{234}}
{{13}{14}{23}{24}{34}}
{{12}{13}{14}{23}{24}{34}}
(End)
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CROSSREFS
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Cf. A000372, A003182, A006126 (labeled case), A007411, A014466, A261005, A293993, A304997, A304998, A304999, A305001, A305855, A306505, A320449, A321679.
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KEYWORD
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nonn,nice,hard
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AUTHOR
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EXTENSIONS
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a(6) from A. Boneh, 32 Hantkeh St., Haifa 34608, Israel, Mar 31 2000
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STATUS
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approved
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