We show that the classical fourth order accurate compact finite difference scheme with high order strong stability preserving time discretizations for convection diffusion problems satisfies a weak monotonicity property, which implies that a simple limiter can enforce the bound- preserving property without losing conservation and high order accuracy. Higher order accurate compact finite difference schemes satisfying the weak monotonicity will also be discussed.

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We begin by giving some results of continuity with respect to the domain for the Dirichlet problem (without any assumption of regularity on the domains). Then, following an idea of A. Beurling, a technique of subsolutions and supersolutions for the so-called quadrature surface free boundary problem is presented. This technique would apply to many free boundary problems in$R$$N$,$N$≥2, which have overdetermined Cauchy data on the free boundary. Some applications to concrete examples are also given.

Let$I$be a union of finitely many closed intervals in [−1, 0). Let$I$^{↞}be a single interval of the form [−1, −a] chosen to have the same logarithmic length as$I$. Let$D$be the unit disc. Then, Beurling [8] has shown that the harmonic measure of the circle ∂$D$at the origin in the slit disc$D$/$I$is increased if$I$is replaced by$I$^{↞}. We prove a number of cognate results and extensions. For instance, we show that Beurling's result remains true if the intervals in$I$are not just one-dimensional, but if they in fact constitute polar rectangles centred on the negative real axis and having some fixed constant angular width. In doing this, we obtain a new proof of Beurling's result. We also discuss a conjecture of Matheson and Pruss [25] and some other open problems.