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In [[numerical mathematics]], '''hierarchical matrices (H-matrices)'''
<ref name="HA99">{{cite journal|last=Hackbusch|first=Wolfgang|date=1999|title=A sparse matrix arithmetic based on H-matrices. Part I: Introduction to H-matrices|journal=Computing|volume=62|pages=89-108}}</ref>
<ref name="GRHA02">{{cite journal|last=Grasedyck|first=Lars|last2=Hackbusch|first2=Wolfgang|date=2003|title=Construction and arithmetics of H-matrices|journal=Computing|volume=70|pages=295-334|url=http://dx.doi.org/10.1007/s00607-003-0019-1}}</ref>
<ref name="HA09">{{cite book|last=Hackbusch|first=Wolfgang|date=2015|title=Hierarchical matrices: Algorithms and Analysis|publisher=Springer|url=http://dx.doi.org/10.1007/978-3-662-47324-5}}</ref>
''Hierarchical matrices: A means to efficiently solve elliptic boundary value problems'',▼
are used as data-sparse approximations of non-sparse matrices.
While a [[sparse matrix]] of dimension <math>n</math> can be represented efficiently in <math>O(n)</math> units of storage
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parameter controlling the accuracy of the approximation.
In typical applications, e.g., when discretizing integral equations
▲
<ref name="MB00">{{cite journal|last=Bebendorf|first=Mario|title=Approximation of boundary element matrices|date=2000|journal=Num. Math.|volume=86|pages=565-589}}</ref>
<ref name="BERJ03">{{cite journal|last=Bebendorf|first=Mario|last2=Rjasanow|first2=Sergej|date=2003|title=Adaptive low-rank approximation of collocation matrices|journal=Computing|volume=70|pages=1-24}}</ref>
<ref name="BOGR05">{{cite journal|last=Börm|first=Steffen|last2=Grasedyck|first2=Lars|date=2005|title=Hybrid cross approximation of integral operators|journal=Num. Math.|volume=101|pages=221-249}}</ref>
<ref name="BOCH16">{{cite journal|last=Börm|first=Steffen|last2=Christophersen|first2=Sven|date=2016|title=Approximation of integral operators by Green quadrature and nested cross approximation|journal=Num. Math.|volume=133|pages=409-442|url=http://dx.doi.org/10.1007/s00211-015-0757-y}}</ref>,
or solving elliptic partial differential equations
<ref name="BO10">{{cite journal|last=Börm|first=Steffen|date=2010|title=Approximation of solution operators of elliptic partial differential equations by H- and H<sup>2</sup>-matrices|journal=Num. Math.|volume=115|pages=165-193|url=http://dx.doi.org/10.1007/s00211-009-0278-7}}</ref>
▲''Existence of H-matrix approximants to the inverse FE-matrix of elliptic operators with <math>L^\infty</math>-coefficients'',
<ref name ="FAMEPR13">M. Faustmann, J. M. Melenk and D. Praetorius,
''H-matrix approximability of the inverses of FEM matrices'',
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Compared to many other data-sparse representations of non-sparse matrices, hierarchical matrices offer a major advantage:
the results of matrix arithmetic operations like matrix multiplication, factorization or inversion can be approximated
in <math>O(n k^\alpha\,\log(n)^\beta)</math> operations, where <math>\alpha,\beta\in\{1,2,3\}.</math><ref name="
== Basic idea ==
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