@incollection{AST_1980__75-76__239_0,
     author = {Willems, Jan C.},
     title = {Almost $A(mod \beta)-$ invariant subspaces},
     booktitle = {Analyse des syst\`emes},
     series = {Ast\'erisque},
     pages = {239--248},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {75-76},
     year = {1980},
     mrnumber = {581719},
     zbl = {0459.93029},
     language = {en},
     url = {http://www.numdam.org/item/AST_1980__75-76__239_0/}
}