The paper investigates the nonlinear function spaces introduced by Giaquinta, Modica and Souček. It is shown that a function from ${Cart}^p(\Omega ,{𝐑}^m)$ is approximated by ${𝒞}^1$ functions strongly in ${𝒜}^q(\Omega ,{𝐑}^m)$ whenever $q<p$. An example is shown of a function which is in ${cart}^p(\Omega ,{𝐑}^2)$ but not in ${cart}^p(\Omega ,{𝐑}^2)$.

We study the boundary value problem $-{\mathrm{d}iv\left(\right(\left|\nabla u\right|}^{p_1\left(x\right)-2}+{\left|\nabla u\right|}^{p_2\left(x\right)-2}\left)\nabla u\right)=f(x,u)$ in $\Omega$, $u=0$ on $\partial \Omega$, where $\Omega$ is a smooth bounded domain in ${ℝ}^N$. Our attention is focused on two cases when $f(x,u)=\pm (-\lambda |u{|}^{m\left(x\right)-2}u+\left|u{|}^{q\left(x\right)-2}u\right)$, where $m\left(x\right)=max\{p_1\left(x\right),p_2\left(x\right)\}$ for any $x\in \overline{\Omega }$ or $m\left(x\right)<q\left(x\right)<\frac{N·m\left(x\right)}{(N-m(x\left)\right)}$ for any $x\in \overline{\Omega }$. In the former case we show the existence of infinitely many weak solutions for any $\lambda >0$. In the latter we prove that if $\lambda$ is large enough then there exists a nontrivial weak solution. Our approach relies on the variable exponent theory of generalized Lebesgue-Sobolev spaces, combined with a ${\mathbb{Z}}_2$-symmetric version for even...

We study a nonlinear elliptic system with resonance part and nonlinear boundary conditions on an unbounded domain. Our approach is variational and is based on the well known Landesman-Laser type conditions.

The two-point boundary value problem $$u^{\text{'}\text{'}}+h\left(x\right)u^p=0,a<x<b,u\left(a\right)=u\left(b\right)=0$$ is considered, where $p>1$, $h\in C^1[0,1]$ and $h\left(x\right)>0$ for $a\le x\le b$. The existence of positive solutions is well-known. Several sufficient conditions have been obtained for the uniqueness of positive solutions. On the other hand, a non-uniqueness example was given by Moore and Nehari in 1959. In this paper, new uniqueness results are presented.