@article{Hackbusch_1999,
    doi = {10.1007/s006070050015},
    url = {https://doi.org/10.1007%2Fs006070050015},
    year = {1999},
    month = {apr},
    publisher = {Springer Science and Business Media {LLC}},
    volume = {62},
    number = {2},
    pages = {89--108},
    author = {W. Hackbusch},
    title = {A Sparse Matrix Arithmetic Based on {\textdollar}{\textbackslash}Cal H{\textdollar} -Matrices. Part I: Introduction to {\textdollar}$\lbrace${\textbackslash}Cal H$\rbrace${\textdollar} -Matrices},
    journal = {Computing}
}

@article{Hackbusch_1999, doi = {10.1007/s006070050015}, url = {https://doi.org/10.1007%2Fs006070050015}, year = {1999}, month = {apr}, publisher = {Springer Science and Business Media {LLC}}, volume = {62}, number = {2}, pages = {89--108}, author = {W. Hackbusch}, title = {A Sparse Matrix Arithmetic Based on {\textdollar}{\textbackslash}Cal H{\textdollar} -Matrices. Part I: Introduction to {\textdollar}$\lbrace${\textbackslash}Cal H$\rbrace${\textdollar} -Matrices}, journal = {Computing} }