@article{RSMUP_1989__81__193_0,
     author = {Tortorelli, V. M.},
     title = {A characterization of internal sets},
     journal = {Rendiconti del Seminario Matematico della Universit\`a di Padova},
     pages = {193--200},
     publisher = {Seminario Matematico of the University of Padua},
     volume = {81},
     year = {1989},
     mrnumber = {1020193},
     zbl = {0686.03039},
     language = {en},
     url = {http://archive.numdam.org/item/RSMUP_1989__81__193_0/}
}