@article{CMUC_1977__18_1_a3,
     author = {Iv\'anek, Ji\v{r}{\'\i}},
     title = {The non-axiomatizability of the observational predicate calculus (generalized {Trachtenbrot's} theorems)},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {41--57},
     publisher = {mathdoc},
     volume = {18},
     number = {1},
     year = {1977},
     mrnumber = {0476476},
     zbl = {0398.03011},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1977__18_1_a3/}
}