| Contents | 6 |
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| Preface | 10 |
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| Critical Points of Functionals | 14 |
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| 1.1 Introduction | 14 |
| 1.2 Extrema | 15 |
| 1.3 PalaisÒSmale sequences | 15 |
| 1.4 Cerami sequences | 16 |
| 1.5 Linking sets | 16 |
| 1.6 Previous definitions of linking | 17 |
| 1.7 Notes and remarks | 18 |
| Minimax Systems | 19 |
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| 2.1 Introduction | 19 |
| 2.2 Definitions and theorems | 20 |
| 2.3 Linking subsets | 21 |
| 2.4 A variation | 23 |
| 2.5 Weaker conditions | 24 |
| 2.6 Some consequences | 25 |
| 2.7 Notes and remarks | 27 |
| Examples of Minimax Systems | 28 |
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| 3.1 Introduction | 28 |
| 3.2 A method using homeomorphisms | 28 |
| 3.3 A method using metric spaces | 30 |
| 3.4 A method using homotopy-stable families | 30 |
| 3.5 Examples of linking sets | 32 |
| 3.6 Various geometries | 35 |
| 3.7 A sandwich theorem | 37 |
| 3.8 Notes and remarks | 40 |
| Ordinary Differential Equations | 41 |
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| 4.1 Extensions of PicardÌs theorem | 41 |
| 4.2 Estimating solutions | 43 |
| 4.3 Extending solutions | 44 |
| 4.4 The proofs | 45 |
| 4.5 An important estimate | 46 |
| The Method Using Flows | 48 |
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| 5.1 Introduction | 48 |
| 5.2 Theorem 2.4 | 48 |
| 5.3 Theorem 2.12 | 50 |
| 5.4 Theorem 2.14 | 53 |
| 5.5 Theorem 2.21 | 54 |
| Finding Linking Sets | 60 |
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| 6.1 Introduction | 60 |
| 6.2 The strong case | 61 |
| 6.3 The remaining proofs | 63 |
| 6.4 Notes and remarks | 65 |
| Sandwich Pairs | 66 |
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| 7.1 Introduction | 66 |
| 7.2 Criteria | 67 |
| 7.3 Notes and remarks | 70 |
| Semilinear Problems | 71 |
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| 8.1 Introduction | 71 |
| 8.2 Bounded domains | 71 |
| 8.3 Some useful quantities | 77 |
| 8.4 Unbounded domains | 79 |
| 8.5 Further applications | 83 |
| 8.6 Special cases | 88 |
| 8.7 The proofs | 89 |
| 8.8 Notes and remarks | 91 |
| Superlinear Problems | 92 |
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| 9.1 Introduction | 92 |
| 9.2 The main theorems | 93 |
| 9.3 Preliminaries | 95 |
| 9.4 Proofs | 96 |
| 9.5 The parameter problem | 99 |
| 9.6 The monotonicity trick | 104 |
| 9.7 Notes and remarks | 111 |
| Weak Linking | 113 |
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| 10.1 Introduction | 113 |
| 10.2 Another norm | 114 |
| 10.3 Some examples | 118 |
| 10.4 Some applications | 120 |
| 10.5 Notes and remarks | 132 |
| Fucik Spectrum: Resonance | 134 |
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| 11.1 Introduction | 134 |
| 11.2 The curves | 136 |
| 11.3 Existence | 140 |
| 11.4 Notes and remarks | 144 |
| Rotationally Invariant Solutions | 145 |
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| 12.1 Introduction | 145 |
| 12.2 The spectrum of the linear operator | 146 |
| 12.3 The nonlinear case | 148 |
| 12.4 Notes and remarks | 151 |
| Semilinear Wave Equations | 152 |
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| 13.1 Introduction | 152 |
| 13.2 Convexity and lower semi-continuity | 152 |
| 13.3 Existence of saddle points | 155 |
| 13.4 Criteria for convexity | 158 |
| 13.5 Partial derivatives | 159 |
| 13.6 The theorems | 161 |
| 13.7 The proofs | 161 |
| 13.8 Notes and remarks | 164 |
| Type (II) Regions | 165 |
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| 14.1 Introduction | 165 |
| 14.2 The asymptotic equation | 168 |
| 14.3 Local estimates | 170 |
| 14.4 The solutions | 174 |
| 14.5 Notes and remarks | 177 |
| Weak Sandwich Pairs | 178 |
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| 15.1 Introduction | 178 |
| 15.2 Weak sandwich pairs | 179 |
| 15.3 Applications | 185 |
| 15.4 Notes and remarks | 191 |
| Multiple Solutions | 192 |
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| 16.1 Introduction | 192 |
| 16.2 Two examples | 192 |
| 16.3 Statement of the theorems | 193 |
| 16.4 Some lemmas | 195 |
| 16.5 Local linking | 207 |
| 16.6 The proofs | 209 |
| 16.7 Notes and remarks | 210 |
| Second-Order Periodic Systems | 213 |
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| 17.1 Introduction | 213 |
| 17.2 Proofs of the theorems | 215 |
| 17.3 Nonconstant solutions | 221 |
| 17.4 Notes and remarks | 226 |
| Bibliography | 228 |
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| Index | 239 |