@article{AM2_1992__43_1_137465,
     author = {Janusz Matkowski},
     title = {Functional inequality characterizing nonnegative concave functions in (0, ?)k.},
     journal = {Aequationes mathematicae},
     pages = {219--224},
     publisher = {mathdoc},
     volume = {43},
     number = {1},
     year = {1992},
     zbl = {0756.39017},
     url = {http://geodesic.mathdoc.fr/item/AM2_1992__43_1_137465/}
}