@article{AM2_1978__17_136724,
     author = {Marvin Marcus},
     title = {Pencils of real symmetric matrices and the numerical range.},
     journal = {Aequationes mathematicae},
     pages = {91--103},
     year = {1978},
     volume = {17},
     zbl = {0386.15010},
     url = {http://geodesic.mathdoc.fr/item/AM2_1978__17_136724/}
}