Harnack's principle

In complex analysis, Harnack's principle or Harnack's theorem is one of several closely related theorems about the convergence of sequences of harmonic functions, that follow from Harnack's inequality.

If the functions u₁(z), u₂(z), ... are harmonic in an open connected subset G of the complex plane C, and

$u_1(z) \le u_2(z) \le ...$

in every point of G, then the limit

$\lim_{n\to\infty}u_n(z)$

either is infinite in every point of the domain G or it is finite in every point of the domain, in both cases uniformly in each compact subset of G. In the latter case, the function

$u(z) = \lim_{n\to\infty}u_n(z)$

is harmonic in the set G.

References

• Kamynin, L.I. (2001), "Harnack theorem", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1556080104, http://www.encyclopediaofmath.org/index.php?title=h/h046620 
• This article incorporates material from Harnack's principle on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.