@article{MZ_1960__73_169842,
     author = {Karel Leeuw},
     title = {On the {Adjoint} {Semigroup} and {Some} {Problems} in the {Theory} of {Approximation.}},
     journal = {Mathematische Zeitschrift},
     pages = {219--234},
     publisher = {mathdoc},
     volume = {73},
     year = {1960},
     zbl = {0094.10501},
     url = {http://geodesic.mathdoc.fr/item/MZ_1960__73_169842/}
}