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Cover design: SPi Publisher ServicesPrinted on acid-free paper springer.com Dedicated to our wives Chiara, Brigitte and Barbara.The cover figure displays the solution of Δ 2 u = f in a rectangle with homogeneous Dirichlet boundary condition for a nonnegative function f with its support concentrated near a point on the left hand side. The dark part shows the region where u < 0.x Preface Preface xi in bounded domains is to understand whether the results available in the simplest case m = 1 can also be proved for any m, or whether the results for m = 1 are special, in particular as far as positivity and the use of maximum principles are concerned. The differential equation ( * ) is complemented with suitable boundary conditions. As already mentioned above, if m = n = 2, equation ( * ) may be considered as a nonlinear plate equation for plates subject to nonlinear feedback forces, one may think e.g. of suspension bridges. In this case, ( * ) may also be interpreted as a reactiondiffusion equation, where the diffusion operator Δ 2 refers to (linearised) surface diffusion.The first part of Chapter 7 is devoted to the proof of symmetry results for positive solutions to ( * ) in the ball under Dirichlet boundary conditions. As already mentioned, truncation and reflection methods do not apply to higher order problems so that a suitable generalisation of the moving planes technique is needed here. Equation ( * ) deserves a particular attention when f (u) has a power-type behaviour. In this case, a crucial role is played by the critical power s = (n + 2m)/(n − 2m) which corresponds to the critical (Sobolev) exponent which appears whenever n > 2m. Indeed, subcritical problems in bounded domains enjoy compactness properties as a consequence of the Rellich-Kondrachov embedding theorem. But compactness is lacking when the critical growth is attained and by means of Pohožaev-type identities, this gives rise to many interesting phenomena. The existence theory can be developed similarly to the second order case m = 1 while it becomes immediately quite difficult to prove positivity or nonexistence of certain solutions. Nonexistence phenomena are related to so-called critical dimensions introduced by Pucci-Serrin [348,349]. They formulated an interesting conjecture concerning these critical dimensions. We give a proof of a relaxed form of it in Chapter 7. We also give a functional analytic interpretation of these nonexistence results, which is reflected in the possibility of adding L 2 -remainder terms in Sobolev inequalities with critical exponent and optimal constants. Moreover, the influence of topological and geometrical properties of Ω on the solvability of the equation is investigated. Also applications to conformal geometry, such as the Paneitz-Branson equation, i...