@article{Matkowski:2009,
author
=
"J.Matkowski",
title
=
"Functional inequality characterizing nonnegative concave functions in $(0,\infty)^k$",
journal
=
"Aequationes Mathematicae",
volume
=
"Vol. 43",
number
=
"no 2-3",
pages
=
"219--224",
year
=
"1992",
}