@article{Matkowski:1993,
  author  = "J. Matkowski",
  title   = "Subadditive functions and a relaxation of the homogeneity condition of seminorms",
  journal = "Proceedings of the American Mathematical Society",
  volume  = "Vol. 117",
  number  = "no 4",
  pages   = "991--1001",
  year    = "1993",
}