@article{PORMA_1987__44_2_115574,
     author = {Joyner, David},
     title = {Summation operators and },
     journal = {Portugaliae mathematica},
     pages = {119--130},
     publisher = {mathdoc},
     volume = {44},
     number = {2},
     year = {1987},
     mrnumber = {0911435},
     zbl = {0631.10031},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_1987__44_2_115574/}
}