@article{EXMA_1993__2_2_230043,
     author = {Stevenhagen, Peter},
     title = {The number of real quadratic fields having units of negative norm.},
     journal = {Experimental mathematics},
     pages = {121--136},
     publisher = {mathdoc},
     volume = {2},
     number = {2},
     year = {1993},
     zbl = {0792.11041},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/EXMA_1993__2_2_230043/}
}