@article{MS2_1971__28_166186,
     author = {A. Ehrenfeucht and G. Fuhrken},
     title = {On {Models} with {Undefinable} {Elements.}},
     journal = {Mathematica Scandinavica},
     pages = {325--328},
     publisher = {mathdoc},
     volume = {28},
     year = {1971},
     zbl = {0236.02041},
     url = {http://geodesic.mathdoc.fr/item/MS2_1971__28_166186/}
}