@article{MAN_1970__187_162005,
     author = {Juan Jorge Sch\"affer},
     title = {Linear {Difference} {Equations:} {Closedness} of {Covariant} {Sequences.}},
     journal = {Mathematische Annalen},
     pages = {69--76},
     publisher = {mathdoc},
     volume = {187},
     year = {1970},
     zbl = {0185.22501},
     url = {http://geodesic.mathdoc.fr/item/MAN_1970__187_162005/}
}