Maximal functions related to subelliptic operators invariant under an action of a nilpotent Lie group
On the domain S_a = {(x,e^b): x ∈ N, b ∈ ℝ, b > a} where N is a simply connected nilpotent Lie group, a certain N-left-invariant, second order, degenerate elliptic operator L is considered. N × {e^a} is the Poisson boundary for L-harmonic functions F, i.e. F is the Poisson integral F(xe^b) = ʃ_N f(xy)dμ^b_a(x), for an f in L^∞(N). The main theorem of the paper asserts that the maximal function M^a f(x) = sup{|ʃf(xy)dμ_a^b(y)| : b > a} is of weak type (1,1).
Assume that u, v are conjugate harmonic functions on the unit disc of ℂ, normalized so that u(0) = v(0) = 0. Let u*, |v|* stand for the one- and two-sided Brownian maxima of u and v, respectively. The paper contains the proof of the sharp weak-type estimate ℙ(|v|* ≥ 1)≤ (1 + 1/3² + 1/5² + 1/7² + ...)/(1 - 1/3² + 1/5² - 1/7² + ...) 𝔼u*. Actually, this estimate is shown to be true in the more general setting of differentially subordinate harmonic functions defined...
A positive measurable function K on a domain D in is called a mean value density for temperatures if for all temperatures u on D̅. We construct such a density for some domains. The existence of a bounded density and a density which is bounded away from zero on D is also discussed.
Let be a -subharmonic function with associated measure , and let be a superharmonic function with associated measure , on an open set . For any closed ball , of centre and radius , contained in , let denote the mean value of over the surface of the ball. We prove that the upper and lower limits as with of the quotient , lie between the upper and lower limits as of the quotient . This enables us to use some well-known measure-theoretic results to prove new variants and generalizations...
Soit un compact polynomialement convexe de et son “potentiel logarithmique extrémal” dans . Supposons que est régulier (i.e. continue) et soit une fonction holomorphe sur un voisinage de . On construit alors une suite de polynôme de variables complexes avec deg pour , telle que l’erreur d’approximation soit contrôlée de façon assez précise en fonction du “pseudorayon de convergence” de par rapport à et du degré de convergence . Ce résultat est ensuite utilisé pour étendre...
The aim of this paper is to analyze mathematically the method of fundamental solutions applied to the biharmonic problem. The key idea is to use Almansi-type decomposition of biharmonic functions, which enables us to represent the biharmonic function in terms of two harmonic functions. Based on this decomposition, we prove that an approximate solution exists uniquely and that the approximation error decays exponentially with respect to the number of the singular points. We finally present results...