@article{CMH_1973__48_139540,
     author = {Robert Fittler},
     title = {Categories with {Ultraproducts}},
     journal = {Commentarii mathematici Helvetici},
     pages = {100--115},
     publisher = {mathdoc},
     volume = {48},
     year = {1973},
     zbl = {0262.18002},
     url = {http://geodesic.mathdoc.fr/item/CMH_1973__48_139540/}
}