Inventiones mathematicae

, Volume 187, Issue 3, pp 535–587

On the singularities of a free boundary through Fourier expansion

  • John Andersson
  • Henrik Shahgholian
  • Georg S. Weiss

DOI: 10.1007/s00222-011-0336-5

Cite this article as:
Andersson, J., Shahgholian, H. & Weiss, G.S. Invent. math. (2012) 187: 535. doi:10.1007/s00222-011-0336-5


In this paper we are concerned with singular points of solutions to the unstable free boundary problem
$$\Delta u = - \chi_{\{u>0\}} \quad\hbox{in } B_1.$$
The problem arises in applications such as solid combustion, composite membranes, climatology and fluid dynamics.

It is known that solutions to the above problem may exhibit singularities—that is points at which the second derivatives of the solution are unbounded—as well as degenerate points. This causes breakdown of by-now classical techniques. Here we introduce new ideas based on Fourier expansion of the nonlinearity χ{u>0}.

The method turns out to have enough momentum to accomplish a complete description of the structure of the singular set in ℝ3.

A surprising fact in ℝ3 is that although
can converge at singularities to each of the harmonic polynomials
$$xy,\qquad {x^2+y^2\over2}-z^2\quad \textrm{and}\quad z^2-{x^2+y^2\over2},$$
it may not converge to any of the non-axially-symmetric harmonic polynomials α((1+δ)x2+(1−δ)y2−2z2) with δ≠1/2.

We also prove the existence of stable singularities in ℝ3.

Mathematics Subject Classification (2000)

35R35 35B40 35J60 

Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • John Andersson
    • 1
  • Henrik Shahgholian
    • 2
  • Georg S. Weiss
    • 3
  1. 1.Mathematics InstituteUniversity of WarwickCoventryUK
  2. 2.Department of MathematicsRoyal Institute of TechnologyStockholmSweden
  3. 3.Mathematical Institute of the Heinrich Heine UniversityDüsseldorfGermany

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