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 A006602 a(n) is the number of hierarchical models on n unlabeled factors or variables with linear terms forced. (Formerly M1532) 27
 2, 1, 2, 5, 20, 180, 16143, 489996795 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS 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 The non-spanning/covering case is A003182. The labeled case is A006126. - Gus Wiseman, Feb 20 2019 REFERENCES 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). LINKS V. Jovovic and G. Kilibarda, On the number of Boolean functions in the Post classes F^{mu}_8, Diskretnaya Matematika, 11(4) (1999), 127-138 (translated in Discrete Mathematics and Applications, 9(6) (1999), 593-605). C. Lienkaemper, When do neural codes come from convex or good covers?, 2015. C. L. Mallows, Emails to N. J. A. Sloane, Jun-Jul 1991 FORMULA a(n) = A007411(n) + 1. First differences of A003182. - Gus Wiseman, Feb 23 2019 EXAMPLE From Gus Wiseman, Feb 20 2019: (Start) 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) CROSSREFS Cf. A000372, A003182, A006126 (labeled case), A007411, A014466, A261005, A293993, A304997, A304998, A304999, A305001, A305855, A306505, A320449, A321679. Sequence in context: A031148 A032238 A000619 * A144824 A144358 A049404 Adjacent sequences:  A006599 A006600 A006601 * A006603 A006604 A006605 KEYWORD nonn,nice,hard AUTHOR EXTENSIONS a(6) from A. Boneh, 32 Hantkeh St., Haifa 34608, Israel, Mar 31 2000 Entry revised by N. J. A. Sloane, Jul 23 2006 a(7) from A007411 and A003182. - N. J. A. Sloane, Aug 13 2015 Named edited by Petros Hadjicostas, Apr 08 2020 STATUS approved

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Last modified May 24 18:33 EDT 2020. Contains 334580 sequences. (Running on oeis4.)