:point_right: On October 24 and 31, we covered the following:

  • Boundary behaviour of harmonic functions and the notion of a barrier:
    • if the external ball condition is satisfied at x0Ωx_0\in\partial\Omega, then there exists a (harmonic) barrier at x0x_0;
    • if there exists a barrier at x0x_0, then Poincaré’s méthode de balayage gives a solution of Δu=0\Delta u =0 which is continuous up to the boundary at x0x_0;
  • Sobolev spaces Wm,pW^{m,p} (of functions in LpL^p with weak derivatives up to order mm that are also in LpL^p): two definitions;
  • A few examples;

  • Theorem: Wm,pW^{m,p} is a Banach space;

  • Density of smooth functions in Wm,pW^{m,p}; the Meyers-Serrin theorem `H=WH=W’;

  • Poincaré’s inequality for uW01,pu \in W_0^{1,p} on a bounded domain Ω\Omega;

  • Some remarks on the one dimensional case and on the case p=np=n;

  • Sobolev’s imbedding theorem for 1p<n% <![CDATA[ 1\le p < n %]]> on Rn\mathbb{R}^n, with Gagliardo’s proof:
(Rnupdx)1/pC(n,p)(Rnupdx)1/p,p=npnp;\displaystyle \left(\int_{\mathbb{R}^n} |u|^{p^\ast}\, dx\right)^{1/p^\ast} \le C(n,p) \biggl(\int_{\mathbb{R}^n} |\nabla u|^{p}\, dx\biggr)^{1/p}\, , \qquad p^\ast=\frac{np}{n-p};
  • The estimate of oscillations of a C1C^1 function uu on a convex domain Ω\Omega by Riesz potentials:
u(x)uSdiam(Ω)nnSΩxy1nu(y)dy,xΩ, SΩ;\displaystyle |u(x)-u_S| \le \frac{\text{diam}(\Omega)^n}{n|S|} \int_{\Omega} |x-y|^{1-n}|\nabla u(y)|\, dy\, , \qquad x\in \Omega, \ S\subset \Omega;
  • As a corollary of the above: Morrey’s imbedding of W1,pW^{1,p}, p>np>n, into CαC^\alpha, α=1np\alpha=1-\frac np;
  • As another corollary: in dimension nn, functions of class Wm,pW^{m,p}, where mp>nmp>n, are continuous (upon a modification on a set of measure zero).

You can find this material in Gilbarg and Trudinger (Chapter 2 on harmonic functions and Chapter 7 on Sobolev spaces), Evans (Chapter 5) and my lecture notes in Polish (Chapter 6, Sobolev spaces).