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 , , ... are harmonic in an open connected subset of the complex plane C, and
in every point of , then the limit
either is infinite in every point of the domain or it is finite in every point of the domain, in both cases uniformly in each compact subset of . In the latter case, the function
is harmonic in the set .