@article{AMUC2_2006__75_1_130057,
     author = {Thron, R. and Koppitz, J.},
     title = {Free systems of algebras and ultraclosed classes.},
     journal = {Acta Mathematica Universitatis Comenianae. New Series},
     pages = {127},
     publisher = {mathdoc},
     volume = {75},
     number = {1},
     year = {2006},
     zbl = {1164.08004},
     language = {en},
     url = {https://geodesic-test.mathdoc.fr/item/AMUC2_2006__75_1_130057/}
}