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On Parsimonious Explanations For 2-D Tree- and Linearly-Ordered Data Karloff, Howard Korn, Flip Makarychev, Konstantin Rabani, Yuval ordered data explanation problem This paper studies the ``explanation problem'' for tree- and linearly-ordered array data, a problem motivated by database applications and recently solved for the one-dimensional tree-ordered case. In this paper, one is given a matrix A=(a_{ij}) whose rows and columns have semantics: special subsets of the rows and special subsets of the columns are meaningful, others are not. A submatrix in A is said to be meaningful if and only if it is the cross product of a meaningful row subset and a meaningful column subset, in which case we call it an ``allowed rectangle.'' The goal is to ``explain'' A as a sparse sum of weighted allowed rectangles. Specifically, we wish to find as few weighted allowed rectangles as possible such that, for all i,j, a_ij equals the sum of the weights of all rectangles which include cell (i,j). In this paper we consider the natural cases in which the matrix dimensions are tree-ordered or linearly-ordered. In the tree-ordered case, we are given a rooted tree $T_1$ whose leaves are the rows of $A$ and another, $T_2$, whose leaves are the columns. Nodes of the trees correspond in an obvious way to the sets of their leaf descendants. In the linearly-ordered case, a set of rows or columns is meaningful if and only if it is contiguous. For tree-ordered data, we prove the explanation problem NP-Hard and give a randomized $2$-approximation algorithm for it. For linearly-ordered data, we prove the explanation problem NP-Har and give a $2.56$-approximation algorithm. To our knowledge, these are the first results for the problem of sparsely and exactly representing matrices by weighted rectangles. Schloss Dagstuhl – Leibniz-Zentrum für Informatik Howard Karloff and Flip Korn and Konstantin Makarychev and Yuval Rabani 2011 Is Part Of LIPIcs, Volume 9, 28th International Symposium on Theoretical Aspects of Computer Science (STACS 2011) InProceedings Text doc-type:ResearchArticle publishedVersion application/pdf doi:10.4230/LIPIcs.STACS.2011.332 urn:nbn:de:0030-drops-30246 https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2011.332 eng https://creativecommons.org/licenses/by-nc-nd/3.0/legalcode