On the qualitative theory of parametrized families of free boundaries. Andrew Acker — 1989 Journal für die reine und angewandte Mathematik
Qualitative behavior of axial-symmetric solutions of elliptic free boundary problems. Acker, Andrew F.; Lancaster, Kirk E. — 1997 Electronic Journal of Differential Equations (EJDE) [electronic only]
A free boundary problem for the p -Laplacian: uniqueness, convexity, and successive approximation of solutions. Acker, Andrew F.; Meyer, R. — 1995 Electronic Journal of Differential Equations (EJDE) [electronic only]