@article{MM2_1989__64_2_155415,
     author = {Celso Mart{\'\i}nez and Miguel Sanz},
     title = {N-th roots of a non-negative operator: conditions for uniqueness.},
     journal = {Manuscripta mathematica},
     pages = {403--418},
     publisher = {mathdoc},
     volume = {64},
     number = {2},
     year = {1989},
     zbl = {0684.47014},
     url = {http://geodesic.mathdoc.fr/item/MM2_1989__64_2_155415/}
}