An Extension of De Giorgi Class and Applications

We present an extension of the classical De Giorgi class, and then we show that functions in this new class are locally bounded and locally H\"older continuous. Some applications are given. As a first application, we give a regularity result for local minimizers $u:\Omega \subset \mathbb R^4 \rightarrow \mathbb R^4$ of a special class of polyconvex functionals with splitting form in four dimensional Euclidean spaces. Under some structural conditions on the energy density, we prove that each component $u^\alpha$ of the local minimizer $u$ belongs to the generalized De Giorgi class, then one can derive that it is locally bounded and locally H\"older continuous. Our result can be applied to polyconvex integrals whose prototype is $$\int_\Omega \Big(\sum_{\alpha =1}^4 |Du^\alpha|^p + \sum_{\beta =1}^6 |({\rm adj}_2 Du )^\beta | ^q +\sum_{\gamma =1}^4 |({\rm adj}_3 Du )^\gamma | ^r +|\det Du|^s \Big ) \mathrm {d}x$$ with suitable $p,q,r,s\ge 1$. As a second application, we consider a degenerate linear elliptic equation of the form $$-\mbox {div} (a(x)\nabla u)=-\mbox {div}F,$$ with $0<a(x) \le \beta <+\infty$. We prove, by virtue of the generalized De Giorgi class, that any weak solution is locally bounded and locally H\"older continuous provided that $\frac 1 {a(x)}$ and $F(x)$ belong to some suitable locally integrable function spaces. As a third application, we show that our theorem can be applied in dealing with regularity issues of elliptic equations with non-standard grow conditions. As a fourth application we treat with quasilinear elliptic systems. Under suitable assumptions on the coefficients, we show that any of its weak solutions is locally bounded and locally H\"older continuous.