1. M. Bôcher, A problem in statics and its relation to certain algebraic invariants, Proc. Amer. Acad. Arts Sci. vol. 40 (1904) pp. 469-484.
2. G. Darboux, Sur une classe remarquable de courbes et de surfaces algébriques et sur la théorie des imaginaires, Paris, 1873, pp. 107-334.
3. On the zeros of linear partial fractions;Marden, Morris;Trans. Amer. Math. Soc.,1930
4. On the zeros of the derivative of a rational function;Marden, Morris;Bull. Amer. Math. Soc.,1936
5. \bysame, The geometry of the zeros of a polynomial in a complex variable, New York, 1949.