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In the present article we provide an example of two closed non-$\sigma$-lower porous sets $A,B\subseteq ℝ$ such that the product $A\times B$ is lower porous. On the other hand, we prove the following: Let $X$ and $Y$ be topologically complete metric spaces, let $A\subseteq X$ be a non-$\sigma$-lower porous Suslin set and let $B\subseteq Y$ be a non-$\sigma$-porous Suslin set. Then the product $A\times B$ is non-$\sigma$-lower porous. We also provide a brief summary of some basic properties of lower porosity, including a simple characterization of Suslin non-$\sigma$-lower porous sets in topologically...

For any $\alpha ,\beta >0$ with $\alpha +\beta <1$ we provide a simple construction of an $\alpha$-Hölde function $f:[0,1]\to ℝ$ and a $\beta$-Hölder function $g:[0,1]\to ℝ$ such that the integral ${\int }_0^{1}f\mathrm{d}g$ fails to exist even in the Kurzweil-Stieltjes sense.

We prove a separable reduction theorem for $\sigma$-porosity of Suslin sets. In particular, if $A$ is a Suslin subset in a Banach space $X$, then each separable subspace of $X$ can be enlarged to a separable subspace $V$ such that $A$ is $\sigma$-porous in $X$ if and only if $A\cap V$ is $\sigma$-porous in $V$. Such a result is proved for several types of $\sigma$-porosity. The proof is done using the method of elementary submodels, hence the results can be combined with other separable reduction theorems. As an application we extend a theorem...