@article{TAC_1997_3_a2,
     author = {Michael Barr},
     title = {Note on a  theorem of {Putnam's}},
     journal = {Theory and applications of categories},
     pages = {.45--49
http://www},
     publisher = {mathdoc},
     volume = {3},
     year = {1997},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_1997_3_a2/}
}