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On a problem of Mazur from "The Scottish Book" concerning second partial derivatives

Volodymyr Mykhaylyuk, Anatolij Plichko (2015)

Colloquium Mathematicae

We comment on a problem of Mazur from “The Scottish Book" concerning second partial derivatives. We prove that if a function f(x,y) of real variables defined on a rectangle has continuous derivative with respect to y and for almost all y the function F y ( x ) : = f y ' ( x , y ) has finite variation, then almost everywhere on the rectangle the partial derivative f y x ' ' exists. We construct a separately twice differentiable function whose partial derivative f x ' is discontinuous with respect to the second variable on a set of positive...

On sets of discontinuities of functions continuous on all lines

Luděk Zajíček (2022)

Commentationes Mathematicae Universitatis Carolinae

Answering a question asked by K. C. Ciesielski and T. Glatzer in 2013, we construct a C 1 -smooth function f on [ 0 , 1 ] and a closed set M graph f nowhere dense in graph f such that there does not exist any linearly continuous function on 2 (i.e., function continuous on all lines) which is discontinuous at each point of M . We substantially use a recent full characterization of sets of discontinuity points of linearly continuous functions on n proved by T. Banakh and O. Maslyuchenko in 2020. As an easy consequence of our...

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