Multipliers of nonnegative derivatives (Q1071120)

The paper includes a discussion of properties of the system M of all real functions f defined on the unit interval such that the product fg is a derivative for each nonnegative derivative g. Amont others it is proved that M is closed with respect to uniform convergence and that M is included in the class of Riemann integrable functions. Also there are constructions of functions belonging to M and possessing ''big'' set of points of discontinuity in the sense of cardinality.