viii Contents
2.2 Stability results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
2.2.1 Remarks on a class of test functions . . . . . . . . . . . . . 83
2.2.2 Convergence of maximum points . . . . . . . . . . . . . . . 85
2.2.3 Applications .......................... 87
2.3 Boundary value problems . . . . . . . . . . . . . . . . . . . . . . . 92
2.4 Perron’smethod ............................ 98
2.4.1 Closedness under supremum . . . . . . . . . . . . . . . . . . 100
2.4.2 Maximalsubsolution...................... 101
2.4.3 Adaptation for very singular equations . . . . . . . . . . . . 103
2.4.4 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . 105
2.5 Notesandcomments.......................... 105
3 Comparison principle 109
3.1 Typicalstatements........................... 109
3.1.1 Bounded domains . . . . . . . . . . . . . . . . . . . . . . . 110
3.1.2 Generaldomains ........................ 112
3.1.3 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . 112
3.2 Alternatedefinitionofviscositysolutions............... 113
3.2.1 Definitioninvolvingsemijets.................. 113
3.2.2 Solutions on semiclosed time intervals . . . . . . . . . . . . 119
3.3 General idea for the proof of comparison principles . . . . . . . . . 123
3.3.1 Atypicalproblem ....................... 123
3.3.2 Maximum principle for semicontinuous functions . . . . . . 126
3.4 Proof of comparison principles for parabolic equations . . . . . . . 128
3.4.1 Proof for bounded domains . . . . . . . . . . . . . . . . . . 129
3.4.2 Proof for unbounded domains . . . . . . . . . . . . . . . . . 134
3.5 Lipschitz preserving and convexity preserving properties . . . . . . 139
3.6 Spatially inhomogeneous equations . . . . . . . . . . . . . . . . . . 148
3.6.1 Inhomogeneity in first order perturbation . . . . . . . . . . 148
3.6.2 Inhomogeneityinhigherorderterms............. 150
3.7 Boundary value problems . . . . . . . . . . . . . . . . . . . . . . . 155
3.8 Notesandcomments.......................... 158
4 Classical level set method 163
4.1 Briefsketchofalevelsetmethod................... 163
4.2 Uniqueness of bounded evolutions . . . . . . . . . . . . . . . . . . . 166
4.2.1 Invariance under change of dependent variables . . . . . . . 166
4.2.2 Orientation-free surface evolution equations . . . . . . . . . 171
4.2.3 Uniqueness ........................... 172
4.2.4 Unbounded evolutions . . . . . . . . . . . . . . . . . . . . . 174
4.3 ExistencebyPerron’smethod..................... 175
4.4 Existencebyapproximation...................... 180
4.5 Variouspropertiesofevolutions.................... 182
4.6 Convergencepropertiesforlevelsetequations............ 192