@article{ZNSL_2016_448_a18,
     author = {A. Khvedelidze and A. Torosyan},
     title = {The spectrum and separability of mixed two-qubit $X$-states},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {270--285},
     year = {2016},
     volume = {448},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2016_448_a18/}
}