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$H$-matrix approximation for the operator exponential with applications. Numer. Math (to appear). 
MR 1917366[2] I. P. Gavrilyuk, W. Hackbusch, B. N. Khoromskij: 
$H$-matrix approximation for elliptic solution operators in cylindric domains. East-West J. Numer. Math. 9 (2001), 25–58. 
MR 1839197[3] L. Grasedyck: Theorie und Anwendungen Hierarchischer Matrizen. Doctoral thesis, University Kiel, 2001.
[5] W. Hackbusch, Z. P. Nowak: 
On the fast matrix multiplication in the boundary element method by panel clustering. Numer. Math. 54 (1989), 463–491. 
DOI 10.1007/BF01396324 | 
MR 0972420[6] W. Hackbusch, B. N. Khoromskij: 
A sparse $H$-matrix arithmetic. Part II: Application to multi-dimensional problems. Computing 64 (2000), 21–47. 
MR 1755846[8] W. Hackbusch, B. N. Khoromskij, S. A. Sauter: 
On $H^{2}$-matrices. Lectures on applied mathematics, Hans-Joachim Bungartz, Ronald H. W. Hoppe, Christoph Zenger (eds.), Springer, Berlin, 2000, pp. 9–29. 
MR 1767775[9] W. Hackbusch, B. N. Khoromskij: 
$H$-matrix approximation on graded meshes. The Mathematics of Finite Elements and Applications X, MAFELAP 1999, John R. Whiteman (ed.), Elsevier, Amsterdam, 2000, pp. 307–316. 
MR 1801984