Part 1.Elliptic Problems 3
Chapter 1.An Introductory Problem 3
1.1.Introduction and heuristic considerations 3
1.2.A one-phase singular perturbation problem 6
1.3.The free boundary condition 17
Chapter 2.Viscosity Solutions and Their Asymptotic Developments 25
2.1.The notion of viscosity solution 25
2.2.Asymptotic developments 27
2.3.Comparison principles 30
Chapter 3.The Regularity of the Free Boundary 35
3.1.Weak results 35
3.2.Weak results for one-phase problems 36
3.3.Strong results 40
Chapter 4.Lipschitz Free Boundaries Are C1,γ 43
4.1.The main theorem.Heuristic considerations and strategy 43
4.2.Interior improvement of the Lipschitz constant 47
4.3.A Harnack principle.Improved interior gain 51
4.4.A continuous family of R-subsolutions 53
4.5.Free boundary improvement.Basic iteration 62
Chapter 5.Flat Free Boundaries Are Lipschitz 65
5.1.Heuristic considerations 65
5.2.An auxiliary family of functions 70
5.3.Level surfaces of normal perturbations of ε-monotone functions 72
5.4.A continuous family of R-subsolutions 74
5.5.Proof of Theorem 5.1 76
5.6.A degenerate case 80
Chapter 6.Existence Theory 87
6.1.Introduction 87
6.2.u+is locally Lipschitz 90
6.3.u is Lipschitz 91
6.4.u+is nondegenerate 95
6.5.u is a viscosity supersolution 96
6.6.u is a viscosity subsolution 99
6.7.Measure-theoretic properties of F(u) 101
6.8.Asymptotic developments 103
6.9.Regularity and compactness 106
Part 2.Evolution Problems 111
Chapter 7.Parabolic Free Boundary Problems 111
7.1.Introduction 111
7.2.A class of free boundary problems and their viscosity solutions 113
7.3.Asymptotic behavior and free boundary relation 116
7.4.R-subsolutions and a comparison principle 118
Chapter 8.Lipschitz Free Boundaries: Weak Results 121
8.1.Lipschitz continuity of viscosity solutions 121
8.2.Asymptotic behavior and free boundary relation 124
8.3.Counterexamples 125
Chapter 9.Lipschitz Free Boundaries: Strong Results 131
9.1.Nondegenerate problems: main result and strategy 131
9.2.Interior gain in space (parabolic homogeneity) 135
9.3.Common gain 138
9.4.Interior gain in space (hyperbolic homogeneity) 141
9.5.Interior gain in time 143
9.6.A continuous family of subcaloric functions 149
9.7.Free boundary improvement.Propagation lemma 153
9.8.Regularization of the free boundary in space 157
9.9.Free boundary regularity in space and time 160
Chapter 10.Flat Free Boundaries Are Smooth 165
10.1.Main result and strategy 165
10.2.Interior enlargement of the monotonicity cone 168
10.3.Control of uv at a “contact point” 172
10.4.A continuous family of perturbations 174
10.5.Improvement of ε-monotonicity 177
10.6.Propagation of cone enlargement to the free boundary 180
10.7.Proof of the main theorem 183
10.8.Finite time regularization 185
Part 3.Complementary Chapters: Main Tools 191
Chapter 11.Boundary Behavior of Harmonic Functions 191
11.1.Harmonic functions in Lipschitz domains 191
11.2.Boundary Harnack principles 195
11.3.An excursion on harmonic measure 201
11.4.Monotonicity properties 203
11.5.ε-monotonicity and full monotonicity 205
11.6.Linear behavior at regular boundary points 207
Chapter 12.Monotonicity Formulas and Applications 211
12.1.A 2-dimensional formula 211
12.2.The n-dimensional formula 214
12.3.Consequences and applications 222
12.4.A parabolic monotonicity formula 230
12.5.A singular perturbation parabolic problem 233
Chapter 13.Boundary Behavior of Caloric Functions 235
13.1.Caloric functions in Lip(1, 1/2) domains 235
13.2.Caloric functions in Lipschitz domains 241
13.3.Asymptotic behavior near the zero set 248
13.4.ε-monotonicity and full monotonicity 256
13.5.An excursion on caloric measure 262
Bibliography 265
Index 269