@article{ELMA_1990__45_6_141491,
     author = {E.S. Freidkin and S.A. Freidkin},
     title = {On a problem by {Shapiro.}},
     journal = {Elemente der Mathematik},
     pages = {137--139},
     publisher = {mathdoc},
     volume = {45},
     number = {6},
     year = {1990},
     url = {http://geodesic.mathdoc.fr/item/ELMA_1990__45_6_141491/}
}