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Hypersurfaces with Mean Curvature Given by an Ambient Sobolev Function

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Hypersurfaces with Mean Curvature Given by an Ambient Sobolev Function j. differential geometry 58 (2001) 371-420 HYPERSURFACES WITH MEAN CURVATURE GIVEN BY AN AMBIENT SOBOLEV FUNCTION REINER SCHATZLE¨ Abstract We consider n-hypersurfaces Σj with interior Ej whose mean curvature are 1,p n+1 given by the trace of an ambient Sobolev function uj ∈ W (R ) (0.1) H = u ν on Σ , Σj j Ej j where ν denotes the inner normal of Σ .We investigate ( 0.1) when Σ → Ej j j Σ weakly as varifolds and prove that Σ is an integral n-varifold with bounded first variation which still satisfies (0.1) for uj → u, Ej → E. p has to satisfy 1 p> (n +1) 2 and p ≥ 4 if n = 1.The difficulty is that in the limit several layers can 3 meet at Σ which creates cancellations of the mean curvature. 1. Introduction ∗ 1.1. The problem. We consider the reduced boundaries ∂ Ej of n+1 sets Ej ⊆ Ω of finite perimeter in an open set Ω ⊆ R ,n ≥ 1.We define the corresponding unit-density n-varifolds ∗ n ∗ (1.1) Vj := v(∂ Ej, 1),µVj = H ∂ Ej = |∇χEj | in Ω, with support ∗ (1.2) Σj := spt Vj = spt µVj = ∂ Ej ∩ Ω. Received July 28, 1999. The author was supported by the DFG-grant SCHA 809/1-1. 371 372 reiner schatzle¨ ∗ The mean curvature of Vj is assumed to be given by the trace on ∂ Ej of a function uj, defined in the ambient space ∗ (1.3) HVj = ujνEj on ∂ Ej, ∇χ Ej ∗ where νEj = denotes the inner normal of ∂ Ej. This is assumed |∇χEj | to hold in a weak sense of the form 1 (1.4) δVj(η)= divV ηdµV = div(ujη)χEj for η ∈ C0 (Ω). Ω Ω We assume that 1,p (1.5) uj ∈ W (Ω) with 1 4 (1.6) (n +1)<p<(n + 1) and p ≥ if n =1. 2 3 We specify the exponent of the Sobolev trace mapping on the boundary n +1 n 1 − =: − . p s (1.6) determines its range as n<s<∞,s≥ 2. 4 In particular, we have imposed p ≥ 3 if n = 1to get s ≥ 2. We put n α := 1 − ∈ ]0, 1[. s If Σj are smooth then (1.3), when satisfied in the classical way, im- plies (1.4) via integration by parts. Conversely, we will see below in Theorem 1.3 that (1.4) implies H ∈ Ls (µ ) and (1.3). Vj loc Vj We impose the bounds (1.7) uj W 1,p(Ω), |∇χEj | = µVj (Ω) ≤ Λ, Ω ambient sobolev function 373 for some Λ < ∞ and want to investigate the limit behaviour of (1.3) when 1,p uj → u weakly in W (Ω), 1 χEj → χE strongly in L (Ω), Vj → V weakly as varifolds. We put Σ:=sptV = spt µV . Our main theorem states that (1.3) still holds in the limit. For notions in geometric measure theory and in BV-context, we refer to [13], [14], [16] and [33]. Theorem 1.1. Under the above assumptions, V is an integral n-varifold in Ω ⊆ Rn+1 with locally bounded first variation and s (1.8) HV ∈ Lloc(µV ). E is a set of finite perimeter in Ω and its reduced boundary is contained in the support Σ of V that is (1.9) ∂∗E ⊆ Σ = spt V. The mean curvature vector satisfies (1.10) HV = uνE µV -almost everywhere on Σ, ∇χE ∗ where νE = denotes the generalized normal of ∂ E, which is put |∇χE| equal to 0 outside of ∂∗E. Equation (1.9) is obtained from the lower semicontinuity of the n+1 n+1 perimeter observing for B (x0) ⊂⊂ Ω with µV (∂B (x0)) = 0 that |∇χE|≤lim inf |∇χE | j→∞ j n+1 n+1 B (x0) B (x0) n+1 n+1 = lim µV (B (x0)) = µV (B (x0)). j→∞ j We remark that the closure of the reduced boundary of E does not have to coincide with the support Σ of V . This is due to cancellations when 374 reiner schatzle¨ parts of Σj meet which have opposite normals. Then the respective parts of Σ do not separate its interior from its exterior, but can instead be considered as hidden boundary of E. The result of these cancellations depend on the number how many layers of Σj meet to one layer on Σ. In Theorem 1.1, two different limit procedures are involved which are not immediately compatible: we pass to the limit for Σj in the sense of varifolds, but for Ej we pass to the limit in the sense of currents. The reduced boundary of Ej can be considered as an n-current ∗ (1.11) ∂ Ej = ∂[[ Ej]] as well. We pass to the limit in (1.4), on the left side with convergence of varifolds, on the right with the convergence of currents which is given 1 by χEj → χE strongly in L (Ω), and get 1 (1.12) δV (η)= div(uη)χE for η ∈ C0 (Ω). Ω At this stage, we assume that V is integral, has bounded first variation s s with HV ∈ Lloc(µV ) and that u ∈ Lloc(|∇χE|). This will be justified later. Under these assumptions, we obtain (1.13) HV µV = u ∇χE. n Since s>n, we get from [33] Corollary 17.8 that θ (µV ) is upper n n semicontinuous. Therefore µV = θ (µV )H Σ for the n-dimensional n n ∗ density θ (µV ) ∈ N of µV and ∇χE = νE H ∂ E. Hence, we conclude that n ∗ θ (µV ) HV = uνE µV -almost everywhere on ∂ E (1.14) and ∗ HV =0 µV -almost everywhere on Σ − ∂ E. n Clearly, the density θ (µV ) corresponds to the number of layers which meet at Σ. We distinguish three cases: n (i) θ (µV )=1. Here Σ separates E from its complement and Σ = ∂∗E. In this case, (1.14) already provides a proof of (1.10), of course, so far s under the assumption that HV ∈ Lloc(µV ). ambient sobolev function 375 n (ii) θ (µV ) = 1is odd. For example if uj,u≥ 0 the picture could look like . ∗ Since Σj = ∂ Ej has interior Ej, the layers which meet have op- posite mean curvature vector. Therefore one expects also cancella- tions for the mean curvature. For three layers, (1.14) gives 1 H = uν . V 3 E On the other hand, the opposite layers create an obstacle for each other if the mean curvature is not equal to zero. Actually, we will prove (1.10) by showing that HV ,u=0, n when θ (µV ) = 1is odd. n (iii) θ (µV )iseven. Here Σ does not separate E from its complement and ∂∗E = ∅.It can be considered as a hidden boundary. (1.14) shows HV =0, in coincidence with (1.10). No claim is made whether u =0on Σ. Actually, this cannot be proved in dimensions n ≥ 2asthe following example communicated by Karsten Große-Brauckmann; see [17] and [18] Theorem 2, shows. This is a one parameter family of surfaces with constant mean curvature equal to H ≡ 1. These 376 reiner schatzle¨ surfaces can periodically be extended to complete surfaces when reflected at the planes vertical at the hexagon edges. Then they converge to a double-density plane. We summarize (i)–(iii) in the following supplement to Theorem 1.1. Theorem 1.2. In the situation of Theorem 1.1, we get for n µV -almost all x ∈ Σ with integral density θ0 := θ (µV ,x) ∈ N that: (i) If θ0 =1then HV (x)=u(x) νE(x). (ii) If θ0 =1is odd, then HV (x),u(x)=0. n+1 1 Moreover, in both cases (i) and (ii), θ (E,x)= 2 . (iii) If θ0 is even, then HV (x)=0, and either θn+1(E,x)=1 and u(x) ≤ 0, or θn+1(E,x)=0 and u(x) ≥ 0. The main task for proving Theorem 1.1 is to establish (1.10). In order to do this, we will approximate V locally by a C1,1-graph M near points x0 ∈ Σ which have a tangent plane Tx0 V = θ0P , for some n-hyperplane P ∈ G(n +1,n), and which have full density n n+1 µV ([θ (µV )=θ0] ∩ B (x0)) lim n+1 =1 ↓0 µV (B (x0)) with respect to µV in the set of points which have the same n-dimensional n 1,1 density θ (µV )asx0. The C -approximation has to be of second order in the sense that not only TV = TM, but also (1.15) HV = HM ambient sobolev function 377 on a set with positive density near x0. Such a second order approxima- tion seems only possible if the varifold itself behaves in a second order way on a reasonably large set, more precisely, if the tilt-excess of V , defined for any T ∈ G(n +1,n)by −n 2 tiltexV (x0,,T):= TxV − T dµV (x), n+1 B (x0) has a quadratic decay 2 (1.16) tiltexV (x0,,Tx0 V )=Ox0 ( ). Tilt-excess decay estimates for varifolds, or more precisely height-excess decay estimates, where −n−2 2 heightexV (x0,,T):= |dist(x − x0,T)| dµV (x), n+1 B (x0) were established in Allard’s Regularity Theorem for unit-density, see [1] Theorem 8.16 where it is proved without specifying the respectives planes that 2α heightexV (x0,τ) ≤ τ heightexV (x0,) and were extended to the higher density case in [3] Theorem 5.6 to 2 heightexV (x0,τ) ≤ Cτ heightexV (x0,). Moreover, it is stated in [3] on p.
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