@article{MET_1985__32_175984,
     author = {Gerd Ronning},
     title = {On the {Nonnegativity} of {XX*} and its {Relevance} in {Econometrics.}},
     journal = {Metrika},
     pages = {35--47},
     year = {1985},
     volume = {32},
     zbl = {0558.62099},
     url = {http://geodesic.mathdoc.fr/item/MET_1985__32_175984/}
}