Let $u$ be a $\delta$-subharmonic function with associated measure $\mu$, and let $v$ be a superharmonic function with associated measure $\nu$, on an open set $E$. For any closed ball $B(x,r)$, of centre $x$ and radius $r$, contained in $E$, let $ℳ(u,x,r)$ denote the mean value of $u$ over the surface of the ball. We prove that the upper and lower limits as $s,t\to 0$ with $0<s<t$ of the quotient $\left(ℳ\right(u,x,s)-ℳ(u,x,t\left)\right)/\left(ℳ\right(v,x,s)-ℳ(v,x,t\left)\right)$, lie between the upper and lower limits as $r\to 0+$ of the quotient $\mu \left(B\right(x,r\left)\right)/\nu \left(B\right(x,r\left)\right)$. This enables us to use some well-known measure-theoretic results to prove new variants...

The classical Whitney extension theorem states that every function in Lip$(\beta ,F)$, $F\subset {\mathbf{R}}^n$, $F$ closed, $k\&lt;\beta \le k+1$, $k$ a non-negative integer, can be extended to a function in Lip$(\beta ,{\mathbf{R}}^n)$. Her Lip$(\beta ,F)$ stands for the class of functions which on $F$ have continuous partial derivatives up to order $k$ satisfying certain Lipschitz conditions in the supremum norm. We formulate and prove a similar theorem in the $L^p$-norm. The restrictions to ${\mathbf{R}}^d$, $d\&lt;n$, of the Bessel potential spaces in ${\mathbf{R}}^n$ and the Besov or generalized Lipschitz...

Given a positive measure μ in ${ℝ}^n$, there is a natural variant of the noncentered Hardy-Littlewood maximal operator $M_{\mu }f\left(x\right)=su{p}_{x\in B}1/\mu \left(B\right){ʃ}_B\left|f\right|d\mu$, where the supremum is taken over all balls containing the point x. In this paper we restrict our attention to rotation invariant, strictly positive measures μ in ${ℝ}^n$. We give some necessary and sufficient conditions for $M_{\mu }$ to be bounded from $L^1\left(d\mu \right)$ to $L^{1,\infty }\left(d\mu \right)$.

Let $\mu$ be a nonnegative Radon measure on ${ℝ}^d$ which only satisfies $\mu \left(B(x,r)\right)\le C_0{r}^n$ for all $x\in {ℝ}^d$, $r>0$, with some fixed constants $C_0>0$ and $n\in (0,d].$ In this paper, a new characterization for the space $\mathrm{RBMO}\left(\mu \right)$ of Tolsa in terms of the John-Strömberg sharp maximal function is established.