In this paper we consider the space R2 with the river metric d* and different types of convexity of this space. We define W-convex structure in (R2, d*) and we give the complete characterization of the convex sets in this space. We consider some measures of noncompactness and we give the moduli of noncompactness for considered measures on this space.

In this paper we consider some metrical and topological properties of the river metric $d^*$ in the plane $\mathbb{R}^2^2$. We give the form of the metric segment and the set of all points that are equidistant from two points in $(\rR^2,d^*)$. We also give the characterization of a compact sets in this space.

We deal with quadratic metric-affine gravity (QMAG), which is an alternative theory of gravity and present a new explicit representation of the field equations of this theory. In our previous work we found new explicit vacuum solutions of QMAG, namely generalised pp-waves of parallel Ricci curvature with purely tensor torsion. Here we do not make any assumptions on the properties of torsion and write down our field equations accordingly. We present a review of research done thus far by several authors in finding new solutions of QMAG and different approaches in generalising pp-waves. We present two conjectures on the new types of solutions of QMAG which the ansatz presented in this paper will hopefully enable us to prove.

We consider the modulus of noncompact convexity $\Delta_{X,\phi}(\varepsilon)$ associated with the minimalizable measure of noncompactness $\phi$. We present some properties of this modulus, while the main result of this paper is showing that $\Delta_{X,\phi }(\varepsilon)$ is a subhomogenous and continuous function on $[0,\phi (\bar{B}_X))$ for an arbitrary minimalizable measure of compactness $\phi$ in the case of a Banach space $X$ with the Radon-Nikodym property.

In this paper we consider modulus of noncompact convexity ΔX,φ associated with the strictly minimalizable measure of noncompactness φ. We also give some its properties and show its continuity on the interval [0, φ(BX)).

In the present paper we give some propositions about conditions for compactness and condensation of the nonlinear superposition operator (1) in lp,σ spaces.