BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS TO ENFORCE LOCAL CONSTRAINTS

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1 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS TO ENFORCE LOCAL CONSTRAINTS G. BAL 1 AND M. COURDURIER 2 Abstract. We present a constructive method to devise boundary conditions for solutions of second-order elliptic equations so that these solutions satisfy specic qualitative properties such as: (i) the norm of the gradient of one solution is bounded from below by a positive constant in the vicinity of a nite number of prescribed points; and (ii) the determinant of gradients of n solutions is bounded from below in the vicinity of a nite number of prescribed points. Such constructions nd applications in recent hybrid medical imaging modalities. The methodology is based on starting from a controlled setting in which the constraints are satised and continuously modifying the coecients in the second-order elliptic equation. The boundary condition is evolved by solving an ordinary dierential equation (ODE) dened so that appropriate optimality conditions are satised. Unique continuations and standard regularity results for elliptic equations are used to show that the ODE admits a solution for suciently long times. 1. Introduction Several recent hybrid medical imaging modalities may be recast as systems of nonlinear partial dierential equations with known sources; see, e.g., [1, 3, 4, 6, 16, 22, 24] for reference on such modalities. The solution of such systems requires that said sources satisfy specic properties which may often be recast as specic, qualitative properties of solutions of second-order partial dierential equations. In the applications presented in, e.g., [8, 10], solutions of second-order elliptic equations are required to have gradients that do not vanish, at least locally. In other applications described in, e.g., [2, 7, 12, 18, 19], the determinant of the gradients of n solutions in spatial dimension n is required to be bounded away from 0. Such qualitative properties are to be ensured by controlling the boundary conditions of the elliptic solutions. Using theories based on complex geometric optics solutions or on unique continuation principles and Runge approximations, it is 1 Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027; gb2030@columbia.edu. 2 Departamento de Matemáticas, Ponticia Universidad Católica de Chile, Santiago, Chile; mcourdurier@mat.puc.cl. 1

2 2 G. BAL 1 AND M. COURDURIER 2 shown in, e.g., [9, 10, 23] that the qualitative properties are satised for an open set of boundary conditions that is not precisely characterized. This paper presents a methodology to construct boundary conditions such that the qualitative properties are satised locally. To simplify the presentation, we consider the setting of a second-order elliptic equation in divergence form with an arbitrary (elliptic) diusion coecient. Starting from a conguration where the diusion coecient is constant and where boundary conditions can easily be dened so that the qualitative property is satised, we propose to continuously deform the diusion coecient from the constant one to the nal coecient of interest. An ordinary dierential equation (ODE) is then prescribed for the evolution of the boundary condition so that the qualitative property of interest is satised, at least locally in the vicinity of a nite number of points of interest, during the whole homotopy transformation. The qualitative property is recast as an adapted set of constraints. The ODE is tailored so that optimality conditions are met to satisfy the set of constraints. That the ODE solution exists for the whole duration of the homotopy transformation is guaranteed by using a unique continuation principle for solutions to elliptic equations. The whole procedure may be seen as an optimal boundary control method so that the elliptic solutions satisfy appropriate constraints inside the domain. The rest of the paper is structured as follows. The construction of boundary conditions ensuring that the gradient of the solution does not vanish in the vicinity of a given point is introduced in section 2. Section 3 presents the main results of this paper. In Section 4, we describe the optimality conditions that justify our choice of the evolution equation (the ODE) and give an example of a simpler, more naive, construction that does not achieve our objectives. Section 5 contains the proofs of the main results. Section 6 generalizes the construction to other settings including the construction of solutions such that the gradients do not vanish at a nite number of points and the construction of solutions whose gradients form a basis in the vicinity of a nite number of points. 2. Description of the Problem and Formulation of the Method Let X be a bounded domain in R n with boundary X. For a given coecient γ(x) and a xed ˆx X, the goal is to nd a boundary condition ˆf such that u(ˆx) 1, where u is the solution of the equation (γ u) = 0 in X u = ˆf In order to construct such an ˆf we propose an evolution scheme. Namely, for a given γ 0 (x) let γ s := (1 s)γ 0 +sγ, s [0, 1]. For a family of boundary conditions

3 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 3 f s } s [0,1] let u s denote the corresponding solution of (γ s u s ) = 0 in X (P s ) u s = f s The proposed scheme consists in constructing f s } s [0,1] with the property that u s (ˆx) is non-decreasing after choosing γ 0 and f 0 such that u 0 (ˆx) 1; for example γ 0 1 and f 0 (x 1, x 2,..., x n ) = x 1. Thus, ˆf := f1 is a solution of the problem since u 1 (ˆx) 1. To construct f s } s [0,1], let us assume that f s = f 0 + s 0 tdt and denote u s = u s / s and γ s = γ s / s = γ γ 0. Dierentiating (P s ) with respect to s gives the equation (γ s u s) + (γ s u s ) = 0 in X u s = g s The condition that u s (ˆx) is non-decreasing becomes u s (ˆx) u s(ˆx) 0. Such a characterization hints at the construction of f s } s [0,1] by means of an initial value problem. We construct f s, g s : s [0, 1]} as the solution of gs = fs s = F (f s, s) f s s=0 = f 0 for an F satisfying two specic conditions. The rst condition on the functional F is that it guarantees u s (ˆx) u s(ˆx) 0. The second condition on F is that it admits a solution for initial value problem, with initial condition f 0, for all s [0, 1]. In this work, we provide an explicit description of a functional F (Denition 3.3) satisfying those two conditions (Theorems 3.4, 3.7), hence not only solving the original problem, but also providing an explicit method to construct the solution ˆf. 3. Notation, Framework and Main Results 3.1. Notation. The following notation will be used through the paper. Let X be a bounded domain, let X denote its boundary, at x X let ν(x) denote the outer unit normal to X. Let X be the closure of X. The notation C k,α, k N, 0 < α 1 represents Hölder continuity, i.e., k continuous derivatives with the k-th derivative being Hölder continuous of order α; in the case α=1 the k-th derivative is Lipschitz continuous. Let C k,α (Ω) be the space of Hölder continuous functions from Ω into R and write X C k,α to mean that X can be locally represented as the graph of a Hölder continuous function. In C k,α (Ω) the norm of a function f is written

4 4 G. BAL 1 AND M. COURDURIER 2 as f k,α,ω. We use the classical notation for the integrable spaces L p (Ω) and the norm f L p (Ω), p [1, ]. Denote as W k,p (Ω), k N, p [1, ], the Sobolev space of functions with k weak derivatives in L p (Ω). In these spaces, we consider the usual norm that makes them Banach spaces. Let W k,p 0 (Ω) be the completion of C0 (Ω) in W k,p (Ω); see [13] for additional details Hypotheses. The following hypotheses will be assumed throughout this section. We assume that X is a bounded subset in R n n, n N xed. We x p (1, ) n 1 and let α = n p 1 (0, 1). We x k N. p We assume that X is a C k+3,α bounded domain, ˆx X is xed. We assume that γ C k+n+3 (X) and that there exist constants c, C such that 0 < c < γ < C in X. Let γ 0 1 and f 0 (x 1, x 2,..., x n ) = x 1. For s [0, 1] dene γ s := (1 s)γ 0 + sγ Main Results. The rst theorem summarizes classical results and shows that the formal calculations in Section 2 are valid in this setting. Theorem 3.1. Let s f s C 1( [0, 1]; C k+2,α ( X) ). For each s [0, 1] there is a unique solution u s C k+2,α (X) of the equation (γ s u s ) = 0 in X (P s ) u s = f s in X and s u s C 1( [0, 1]; C k+2,α (X) ). Let u s = u s / s, f s = u s / s and γ s = γ s / s = γ γ 0. Then u s satises the equation (P s) (γ s u s) + (γ s u s ) = 0 in X u s = f s in X and d ds( 1 2 u s(ˆx) 2) = u s (ˆx) u s(ˆx). For y R let y = (y ) denote the y-directional derivative in R n. Let s [0, 1]. The following auxiliary problem is crucial in our analysis: (γ s λ) = y δˆx in X (A s ) λ = 0 Here, δˆx is the distribution at ˆx such that X δˆxf(x)dx = f(ˆx). The dependence of λ on s is not written explicitly since it will be clear from the context. Theorem 3.2. The problem (A s ) above has a unique solution λ L p (X) C k+3,α (X \ ˆx}). If U (X \ ˆx}) is compact, then s λ U C([0, 1]; L p (X) C k+3,α (U)).

5 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 5 We proceed to dene an adequate functional F for the initial value problem of f s } s [0,1]. Denition 3.3. Given f C k+2,α ( X) and s [0, 1], let u C k+2,α (X) be the solution of (γ s u) = 0 in X u = f Let λ be the solution of (γ s λ) = u(ˆx) δˆx λ = 0 If u(ˆx) = 0 let µ > 0, otherwise let X µ = λ ((γ γ 0) u). γ s ν 2 L 2 ( X) in X We dene F : C k+2,α ( X) [0, 1] C k+2,α ( X) as 0 if µ 0 F (f, s) := µγ s ( ) if µ 0. ν The functional F satises the required properties. Theorem 3.4. Given f C k+2,α ( X) and s [0, 1], let u C k+2,α (X) be the solution of (γ s u) = 0 in X u = f Let g = F (f, s) and let v be the solution of (γ s v) + (γ s u) = 0 v = g Then u(ˆx) v(ˆx) 0. in X The second property for F requires a strong relationship between the solution of the auxiliary problem (A s ) and its normal derivative at the boundary. In particular, the following injectivity results is needed. Theorem 3.5. Let λ L p (X) C k+3,α (X \ ˆx}) be the solution of (A s ) and let γ s / ν X C k+2,α ( X) be its normal derivative at the boundary. Then [ ] [ ] [ ] λ 0 γ s 0 y = 0. ν X

6 6 G. BAL 1 AND M. COURDURIER 2 This motivates us to regard λ and its normal derivative as functions of y R n, a nite dimensional space. Using the continuous dependence of λ on s, we can recast the previous injectivity Theorem as an apparently stronger result. Corollary 3.6. Let λ be the solution of (A s ) and let γ s / ν X be its normal derivative at the boundary. There exists constants a, b, ρ, η > 0 independent of y R n and independent of s [0, 1], such that ρ y aγ s λ bγ s η y. ν k+2,α, X L p (X) ν L 2 ( X) In particular, for any η > 0, the quantities λ γ s ν L 2 ( X) L p (X) and γ s ν γ s ν L 2 ( X) C k+2,α ( X), as functions of y, are uniformly Lipschitz in y R n : y η}, independently of s [0, 1]. We start at s = 0 with an adequate f 0, γ 0, hence the estimates of the Corollary 3.6 will imply the solvability of the initial value problem for all s [0, 1]. Theorem 3.7. There exists a unique solution s f s in C 1( [0, 1]; C k+2,α ( X) ) of the initial value problem s f s = F (f s, s) f s s=0 = f 0. In summary, for F as in Denition 3.3, the initial value problem admits a solution for s [0, 1] and ˆf = f 1 solves the original problem. 4. Some Aspects about the Construction of F In this section we elaborate on the requirements on F : C k+1,α ( X) [0, 1] C k+1,α ( X) that lead us to Denition 3.3. We start by presenting a simple, naive, and awed construction that exemplies some of the diculties before proceeding to the optimal aspects of Denition 3.3. Let us consider scalings of the initial boundary condition, namely we let f s = φ(s)f 0, where φ C 1 ([0, 1]; R). Let u s be the solution of (γ s u s ) = 0 in X (P s ) u s = φ(s)f 0

7 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 7 Dierentiating with respect to s, u s has to solve (γ s u s) + (γ s u s ) = 0 u s = φ (s)f 0 in X We want to construct φ such that u s (ˆx) u s(ˆx) 0, s [0, 1]. Let v s, w s be the solutions of (γ s v s ) = 0 in X v s = φ (s)f 0 in X and (γ s w s ) + (γ s u s ) = 0 w s = 0 Then u s = v s + w s and (it can be checked that φ(s) 0) in X u s (ˆx) u s(ˆx) = u s (ˆx) v s (ˆx) + u s (ˆx) w s (ˆx) = φ (s) φ(s) u s(ˆx) 2 + u s (ˆx) w s (ˆx). Hence, to have u s (ˆx) u s(ˆx) 0 we essentially need φ to satisfy a condition of the form φ (s) φ(s) = max 0, u s(ˆx) w s (ˆx) }. u s (ˆx) 2 This condition implies the following estimate on φ φ (s) φ(s) C u s k+2,α,x C φ(s), so that φ (s) C φ(s) 2. In general, we cannot obtain any better estimate. Such an estimate guarantees the existence of φ for s in an open subset of [0, 1], but it does not guarantee global existence in [0, 1]. Indeed, the existence of φ for s [0, 1] is equivalent to saying that for all s [0, 1], the solution u s of (γ u s ) = 0 in X u s = f 0 in X satises u s (ˆx) 0. Yet, it is known that critical points of elliptic solutions do occur; see, e.g. [5, 11, 14, 17, 21]. This shows that f s may blow up in nite time if f s is large enough. We thus need to construct f s in such a way that f s remains suciently small. The construction of F provided in Denition 3.3 is obtained by requiring an optimality condition in that sense.

8 8 G. BAL 1 AND M. COURDURIER 2 Theorem 4.1. For f C k+2,α ( X) and s [0, 1] let u be the solution of (γ s u) = 0 in X u = f Let v g denote the solution of (γ s v g ) + (γ s u) = 0 v g = g The construction of F (f, s) in Denition 3.3 is such that Proof. Let in X F (f, s) = arg min g 2 L 2 ( X) : g Ck+2,α ( X) u s (ˆx) v g (ˆx) 0 }. G = g C k+2,α ( X) : u s (ˆx) v g (ˆx) 0 } ĝ = arg min g 2 L 2 ( X) : g G} From Theorems 3.4 and 3.7, F (f, s) G, hence G. Also G is convex and closed in C k+2,α ( X). The objective function g L 2 ( X) is strictly convex and coercive in C k+2,α ( X). The existence of ĝ does not automatically follow from this, because C k+2,α ( X) is not reexive, but if ĝ exists, then it is unique. Theorem 3.2 and Denition 3.3 imply the existence of µ = min(0, µ) 0 and λ L p (X) ( C k+2,α ( X) ) with / ν C k+2,α ( X) satisfying (γ s λ) = u s (ˆx)δˆx in X λ = 0 in X, and F (f, s) = µγ s / ν X µ u s (ˆx) v F (f,s) (ˆx) = 0. These are the Karush-Kuhn-Tucker (KKT) conditions [20] for the problem dening ĝ. The existence of the KKT multipliers λ, µ with the above conditions imply that ĝ = F (f, s), and in particular imply the existence of ĝ. The fact that the KKT conditions in a convex problem imply optimality is easy to check in general. We briey present the calculations in this particular case for concreteness. If F (f, s) = 0, F (f, s) is clearly the element in G of minimal norm. Otherwise, µ < 0 and F (f, s) is such that We recall that for any g G u s (ˆx) v F (f,s) (ˆx) = 0. u s (ˆx) v g (ˆx) 0.

9 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 9 For any g G, multiplication of the equation of v g by λ and integration by parts, gives u s (ˆx) v g (ˆx) = λ (γ s u s ) γ s ν g. X X Subtracting this last expression for g G and F (f, s), we obtain γ s X ν g + γ s X ν F (f, s) = u s(ˆx) v g (ˆx) u s (ˆx) v F (f,s) (ˆx) 0 γ s F (f, s) γ s ν ν g. Since µ < 0 and F (f, s) = µγ s / ν 0 the previous inequality implies µ γ s 2 γ s ν L 2 ( X) X ν g γ s g ν L 2 ( X) L 2 ( X) µγ s g L ν 2 ( X) L 2 ( X) F (f, s) L 2 ( X) g L 2 ( X) proving that F (f, s) is the element in G of minimal L 2 ( X) norm. X In summary, among all the possible choices of F satisfying the non-decreasing norm of the gradient at ˆx, our denition of F (f, s) is the one of minimal L 2 ( X) norm at each s [0, 1]. X 5. Proofs and Intermediate Results 5.1. Proof of Theorem 3.1. In this subsection, let k N xed, 0 < α < 1 xed. Theorem 5.1. Let X be a C k+2,α bounded domain in R n. Let f C k+2,α ( X) and h C k,α (X). Let γ C k+1,α (X) be such that c, C constants for which 0 < c γ(x) C < x X. Then there is a unique solution u C k+2,α (X) of the equation (γ u) = h in X u = f in X, and u satises the following estimate where the constant κ depends only on n, α, c, C and X, ) u k+2,α,x κ ( f k+2,α, X + h k,α,x.

10 10 G. BAL 1 AND M. COURDURIER 2 For the previous Theorem, existence is established in [13, Thm. 6.14, Thm. 6.19, Lem. 6.38] and the estimate is a consequence of [13, Thm. 6.6, Lem. 6.38, Thm. 3.7]. The estimate and the linearity of the problem imply a smooth dependence of u with respect to the boundary condition and the equation coecient. This is stated explicitly as follows. Corollary 5.2. Let X be a C k+2,α bounded domain in R n. For an interval I R let s f s C 1 (I; C k+2,α ( X)) and s h s C 1 (I; C k,α (X)). Let s γ s C 1 (I; C k+1,α (X)) and such that c, C constants for which 0 < c γ s (x) C < x X, s I. If we let u s, s I, be the solutions of (γ s u s ) = h s u s (x) = f s (x) in X in X, then s u s C 1 (I; C k+2 (X)). Letting γ s := γ s / s, f s := f s / s and h s := h s / s, we also get that u s := u s / s satises the equation (γ s u s(x)) + (γ s u s (x)) = h s in X u s(x) = f s(x) ) In addition, for a given ˆx X, we have ( u d ds s (ˆx) 2 = 2 u s (ˆx) u s(ˆx). Proof of Theorem 3.1. It is a direct consequence of Theorems 5.1 and Corollary 5.2. Remark 5.3. Theorem 5.1 and Corollary 5.2 remain true if we replace (γ ) by any uniformly elliptic operator L = a ij xi xj + b i xi with a ij, b i C k,α (X). n 5.2. Proof of Theorem 3.2. In this subsection let k N xed. Let p (1, xed. Let α = (n n ) (0, 1). p For y R let y = (y ), s [0, 1], we study the auxiliary problem. (γ s λ) = y δˆx in X (A s ) λ = 0 n 1 ) Intuitively, the solution λ is a directional derivative of a Green's function, and so it should behave as a Green's function with one degree less of regularity. Among the statements in Theorem 3.2, the uniqueness of λ is the simplest and follows from standard arguments. The continuous dependence of λ on s is the most technical aspect and it will require the explicit construction of the singular part of λ. This construction will also prove the existence and regularity stated in Theorem 3.2. The construction of the singular part of λ is presented in a couple of technical lemmas below. We start by introducing the necessary notation.

11 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 11 Denition 5.4. Let E N. We say that c j } j E is a family of homogeneous polynomials centered at ˆx if each c j is a polynomial formed exclusively by monomials centered at ˆx of total degree j, namely c j (x) = c β,j (x ˆx) β β =j where β N n, β = n i=1 β i, (x ˆx) β = Π n i=1(x i ˆx i ) β i and each c β,j R. We say that c β,j } β =j R are the (nitely many) coecients of c j. Denition 5.5. Let E = N or E = 0, 1, 2,..., n}. Let c j } j E be a family of homogeneous polynomials centered at ˆx with c 0 0. We dene the family of functions v m } m E associated to c j } j E as follows. Let B be an open ball centered in ˆx and containing X. Let g be the solution of g = δˆx in B g = 0 in B. Then dene v 0 := 1 c 0 y g. Let w be the solution of w = v 0 w = 0 Then dene v 1 := 1 c 0 ( c 1 w c 1 v 0 ). in B in B. For 2 m, m E, dene recursively v m as the solution of c 0 v m = m 1 i=0 [ (v i c m i ) (c m i v i )] in B v m = 0 in B. Lemma 5.6. A family v m } m E from Denition 5.5 satises (a) v 0 L p (B) C (B \ ˆx}). (b) d j v 0 W j,p (B) for any d j homogeneous polynomial of degree j centered at ˆx, j N. (c) v 1 W 1,p (B) C (B \ ˆx}). (d) We get in X (c 0 v 0 ) = y δˆx (5.1) m (c m i v i ) = 0, m 1 (5.2) i=0

12 12 G. BAL 1 AND M. COURDURIER 2 Proof. Using the same notation as in Denition 5.5, g is the Green's function at ˆx of the Direchlet problem for the Laplacian in B, hence there is an explicit expression of g; see [13]. Since c 0 is constant and v 0 := 1 c 0 y g, properties (a) and (b) are automatically veried from the explicit expression for v 0 (recall that p (1, n n 1 )). Property (a) and the denition of w imply w W 2,p (B) C (B \ ˆx}); see [13, Thm 9.15]. Then (c) follows from the denition of u 1 and (b). Property (d) is the denition of v m rewritten. The family v m } m E has the following regularity. Lemma 5.7. Let c j } j E, d j } j E be families of homogeneous polynomials centered at ˆx. Let v m } m E be the family of functions associated to c j } j E in Denition 5.5. Then (1) v m W m,p (B) C (B \ ˆx}), m E. (2) d j v m W j+m,p (B), m E, j 1. (3) d j v m W m+j 2,p (B), j 1, m 1. Proof. The proof is by induction. As the base case, from the previous Lemma we already have (1) for m = 0 and (2) for m = 0, j 1. We also have (1) for m = 1. The following steps complete the induction argument. [ ] (1) 0 m < M and (2) j 1, 0 m < M Using the denition of v M, M 1 d j v M = d M 1 j [ (v i c M i ) (c M i v i )] c 0 i=0 [ ] (3) for m = M, j 1. = 1 M 1 [ (d j v i c M i ) v i (d j c M i ) d j (c M i v i )] c 0 i=0 = 1 M 1 [ (d j v i c M i ) v i (d j c M i ) (d j c M i v i ) c 0 i=0 c M i v i d j + 2 (c M i v i d j )] and by the inductions hypotheses each summand in the right hand side is in W j+m 2,p (B). [ ] [ ] For M 1, (1) and (3) for j = 1 (2) for j = 1. We have d 1 v M = 2 (v M d 1 ) + d 1 v M

13 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 13 and by induction hypotheses the right hand side is in W M 1,p (B), hence (Chp. 9, [15]) d 1 v M W M+1,p (B). [ ] [ ] For M 1, (3) for j = J and (2) for 1 j < J (2) for J. We have d J v M = 2 (v M d J ) + d J v M v M d J and by induction hypotheses the right hand side is in W M+J 2,p (B), hence (Chp. 9, [15]) d J v M W M+J,p (B). [ ] [ ] (1) 1 m < M and (2) j 1, 1 m < M (1) for m = M. By denition v M = 1 M 1 [ (v i c M i ) (c M i v i )]. c 0 i=0 By induction hypotheses, form 2 the right hand side is in W M 2,p (B), hence v M W M,p (B). Elliptic regularity and the induction hypotheses also imply v M C (B \ ˆx}). In Denition 5.5 we have an explicit construction of each v m in terms of the polynomials c j. This provides an explicit dependence of each v m in terms of the coecients of the c j 's. Lemma 5.8. Let v m } m E be the family associated to c j } j E. We can write each v m as v m = l I m p l,m e l,m where I m is a nite index set, p l,m } l Im is a family of real valued polynomials evaluated in 1/c 0 } c β,j } β =j,1 j m, but otherwise independent of c j }. And where e l,m } l Im is a family of functions in X independent of c j }, each e l,m satisfying (1),(2),(3) for m of Lemma 5.7. Proof. By induction. True for v 0 from its denition with p 1,0 (1/c 0 ) = 1/c 0 and e 1,0 = y g. For m 1, the linear system dening v m can be written as v m = 1 m 1 c 0 i=0 v m = 1 m 1 c 0 = m 1 i=0 β =m i (c m i v i ) i=0 β =m i l I i c β,m i ((x ˆx) β v i ) c β,m i c 0 p l,i ((x ˆx) β e l,i ).

14 14 G. BAL 1 AND M. COURDURIER 2 Dening e l,m } l Im as the solutions e of the family of equations the result follows. e = ((x ˆx) β e l,i ) for 0 i m 1, β = i, l I i We can now explicitly describe the singular part of the solution λ of (A s ). Theorem 5.9. Let γ C K (B). Let c j } K j=0 form the partial Taylor sum of γ about ˆx, namely γ(x) = K c j (x) + γ K (x) j=0 with γ K (x) = o( x ˆx K ), γ K C K (B). Assume c 0 0 and let v m } K m=1 be the family constructed in Denition 5.5, corresponding to the c j } K j=0. Dene w K L p (B) C (B \ ˆx}) as w K = K v m. m=0 Then there exists h K W K 2,p (B) C (B \ ˆx}) such that (γ w K ) = y δˆx + h K in B. In addition, if U is a compact subset of B \ ˆx}, then w K L p (B) C (U) depends continuously in the coecients of c j } K j=0. Also, h K W K 2,p (B) C (U) depends continuously in γ under C K (B) perturbations. Proof. Let K i γ K i = γ(x) c j (x). j=0 Then γ K i (x) = o( x ˆx K i ), γ K i C K (B). We have (γ w K ) = = = K (γ v m ) m=0 K m=0 K m=0 K m j=0 ([ K m j=0 c j + γ K m ] v m ) (c j v m ) + K (γ K m v m ) m=0

15 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 15 (γ w K ) = K i K (c i j v j ) + (γ K m v m ) i=0 j=0 m=0 = y δˆx + h K where the rst term is simplied using equations (5.1), (5.2) and where h K is dened as K h K := (γ K m v m ). m=0 Then h K W K 2,p (B) C (B\ˆx}) by Lemma 5.7. The continuous dependencies of w K and h K are a consequence of Lemma 5.8 and the denitions of w K, h K. Theorem Let X be a C k+2,α bounded domain in R n. Let γ C k+n+2 (X) be such that c, C constants for which 0 < c γ(x) C < x X. Then there is a solution λ L p (X) C k+2,α (X \ ˆx}) of (γ λ) = y δˆx in X (A) λ = 0 Also, for any compact set U (X \ ˆx}), we have that λ U L p (X) C k+2,α (U) depends continuously in γ under C k+n+2 (X) perturbations. Proof. Let B be a ball centered in ˆx and large enough to contain X. Extend γ as C k+n+2 (B) and let K = k + n + 2. Let c j } K j=0 form the partial Taylor series of γ about ˆx and let w K, h K be as in Theorem 5.9. Since h K W K 2,p (B) then (Chp. 9, [15]) there exists a unique v W K,p (B) solution of (γ v) = h K in B v(x) = 0 in B which depends continuously on h K. By Sobolev embedding, v C k+2,α (B) (recall that α = n n/p) and it depends continuously on h K, hence it depends continuously on γ under C k+n+2 (B) perturbations. Additionally, since [w K + v] X C k+2,α ( X), Theorem 5.1 implies that there is a unique w C k+2,α (X) solution of (γ w) = 0 in X w(x) = w K (x) v(x) in X which depends continuously on [w K + v] X C k+2,α ( X), hence it depends continuously in γ under C k+n+2 (B) perturbations. Finally, λ = (w K X +v X +w) is a solution of (A) with the desired properties.

16 16 G. BAL 1 AND M. COURDURIER 2 Proof of Theorem 3.2. Theorem 3.2 is Theorem 5.10 for k + 1 instead of k Proof of Theorem 3.4. We use the same notation as Denition 3.3 and Theorem 3.4. Proof. We separate in two cases. Case 1. If u(ˆx) = 0 then immediately u(ˆx) v(ˆx) = 0. Case 2. The equations for u, v and λ, plus integration by parts, give u(ˆx) v(ˆx) = λ ((γ γ 0 ) u) µ γ s ν g with g = F (f, s). Recall the denition of µ, X µ = λ ((γ γ 0) u). X γ s ν 2 L 2 ( X) Case 2.1. If µ 0 then g 0 and X λ ((γ γ 0) u) 0, hence u(ˆx) v(ˆx) = X λ ((γ γ 0) u) 0. Case 2.2. If µ 0 then g = µ/ ν and we get u(ˆx) v(ˆx) = Proof of Theorem 3.5. We use the notation of Theorem 3.5. Proof. It is clear that [ y = 0 ] [ λ 0 ] [ γ s / ν X 0 ]. In the opposite direction. Assume γ s / ν X = 0, then λ satises the equation (γ s λ) = 0 in X \ ˆx} λ = 0 in X γ s = 0 ν By unique continuation λ 0 in X \ ˆx}. Since λ L p (x) we conclude λ 0. Finally, if y 0 let ϕ C0 (X) be such that y ϕ(ˆx) 0. Then λ 0 L p (X) since λ (γ X s ϕ) = y ϕ(ˆx) Proof of Corollary 3.6. We start with a lemma about injective linear maps dened over a nite dimensional domain. Lemma Let I R be a closed bounded interval. Let (V, V ) be a normed vector space. Let H s : R n V, s I, be a family of injective linear functionals. Assume lim H t(y) = H s (y), y R n, s I. t I,t s X

17 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 17 Then there exist constants 0 < a, b < such that s I a y H s y V b y, y R n. Proof. Let e i } n i=1 be a basis of R n. Since I s H s (e i ) are continuous and I is compact, max i=1,...,n sup s I H s e i <. Since H s are linear, the existence of b > 0 for the second inequality follows. Assume a > 0 such that the rst inequality holds. By the compactness of I and the linearity of each H t, there exists s I and I t s, together with y t R n t s, y t = 1, y t y s, such that H t y t t s 0. But then (using H t (y s y t ) V b y s y t ) 0 H s y s V H s (y s ) H t (y s ) V + H t (y s y t ) V + H t y t V t s 0. Hence H s y s = 0, contradicting the injectivity of H s since y s = 1. Proof of Corollary 3.6. From Theorem 3.5, the linear maps R n y λ L p (X) and R n y γ s / ν C k+2,α ( X) L 2 ( X) are injective s [0, 1] (λ is the solution of (A s )). From Theorem 3.2, for y R n xed, λ L p (X) and γ s / ν C k+2,α ( X) L 2 ( X) depend continuously on s [0, 1]. Lemma 5.11 then implies that all the quantities y, λ L p (X), γ s / ν k+2,α, X and γ s / ν L 2 ( X) are comparable uniformly s [0, 1]. The last statement of Corollary 3.6 is true for any quotient of two Lipschitz function in a set where the denominator is bounded away from zero Proof of Theorem 3.7. Let us recall the denition of F : C k+2,α ( X) [0, 1] C k+2,α ( X). Given f C k+2,α ( X) and s [0, 1], let u C k+2,α (X) be the solution of (γ s u) = 0 in X u = f Let λ be the solution of (γ s λ) = u(ˆx) δˆx λ = 0 in X If u(ˆx) = 0 let µ > 0, otherwise let X µ = λ ((γ γ 0) u). γ s ν 2 L 2 ( X)

18 18 G. BAL 1 AND M. COURDURIER 2 We dened 0 if µ 0 F (f, s) := µγ s ( ) if µ 0. ν Lemma There exists a constant κ > 0 independent of s [0, 1] such that Proof. F (f, s) k+2,α, X κ f k+2,α, X. When F (f, s) 0 there is nothing to prove. Otherwise (λ 0) F (f, s) k+2,α, X = µγ s ν k+2,α, X λ ((γ γ X 0) u) = γ s ν k+2,α, X X γ s ν 2 L 2 ( X) λ γ s ((γ γ 0 ) u) γ s ν L 2 ( X) λ L p (X) γ s ν L 2 ( X) κ u 2,X κ f k+2,α, X. γ s ν k+2,α, X γ s ν L 2 ( X) ν k+2,α, X γ s ν L 2 ( X) ((γ γ 0 ) u) L p/(p 1) (X) We used Hölder inequality to go from the third to the fourth line. Corollary 3.6 and the boundedness of X to go from the fourth to the fth line, and Theorem 5.1 to go from the fth to the last line. Denition Given η > 0 and s [0, 1] let us dene the set N η,s C k+2,α ( X) as follows, f N η,s if and only if the solution u of the equation (γ s u) = 0 in X u = f in X satises u(ˆx) > η. Lemma Fix η > 0, for f N η,s C k+2,α ( X) and s [0, 1] let u, λ and µ be the ones involved in the denition of F (f, s). Then N η,s f λ/ γ s ν L 2 ( X) L p (X) is Lipschitz continuous and bounded, uniformly in s [0, 1]. N η,s f γ s / γ ν s ν L 2 ( X) C k+2,α ( X) is Lipschitz continuous and bounded, uniformly in s [0, 1].

19 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 19 N η,s f u C k+2,α (X) is linear continuous, uniformly in s [0, 1]. Proof. The last property is a direct consequence of Theorem 5.1. The rst two properties are quickly deduced from Theorem 5.1, the denition of λ and Corollary 3.6. Theorem Given η > 0 there exists κ > 0 such that s [0, 1], f 1, f 2 N η,s F (f 1, s) F (f 2, s) k+2,α, X κ(1 + f 1 k+2,α, X + f 2 k+2,α, X ) f 1 f 2 k+2,α, X. Proof. Let u i, λ i, µ i, i = 1, 2 be the values appearing in the denitions of F (f 1, s) and F (f 2, s) correspondingly. If µ 1, µ 2 0 then F (f 1, s) F (f 2, s) 0 and F (f 1, s) F (f 2, s) k+2,α, X = 0. If µ 1 0 and µ 2 0 then F (f 1, s) = 0 and F (f 1, s) F (f 2, s) k+2,α, X = F (f 2, s) k+2,α, X λ 2 = X γ 2 s ((γ γ 0 ) u 2 ) γ s 2 ν L 2 ( X) γ 2 s λ 2 ρ X γ 2 s ((γ γ 0 ) u 2 ) ν L 2 ( X) λ 2 ρ X γ 2 s ((γ γ 0 ) u 2 ) ν L 2 ( X) λ 1 X γ 1 s ((γ γ 0 ) u 1 ). ν L 2 ( X) ν k+2,α, X ν L 2 ( X) From the second to the third line we used Corolarry 3.6. From the third to the last line we used the fact that each integral has the same sign as the corresponding µ i, and we are in the case of µ i 's with opposite signs. If µ i, µ 2 0 then F (f 1, s) F (f 2, s) k+2,α, X = X λ 2 X γ s 2 ν L 2 ( X) λ 1 γ 1 s ((γ γ 0 ) u 1 ) ν L 2 ( X) ((γ γ 0 ) u 2 ) γ s 2 ν γ s 2 ν L 2 ( X) γ s 1 ν γ 1 s ν L 2 ( X). k+2,α, X Using Lemma 5.14 we observe that in any of the three cases, we are left with products of bounded Lipschitz functions and one continuous linear function, all bounds being uniform in s [0, 1], which readily implies the estimate above. Proof of Theorem 3.7. Let η > 0 be such that f 0 N η,0 and let ρ > 0 be such that F (f, s) k+2, X ρ f k+2, X for all f C k+2 ( X), s [0, 1] (such ρ exists by Lemma 5.12).

20 20 G. BAL 1 AND M. COURDURIER 2 In order to prove Theorem 3.7, it is enough to show that there is f s } s [0,1] C ( [0, 1]; C k+2,α ( X) ), such that s [0, 1], f s = f 0 + s 0 F (f τ, τ)dτ. (5.3) Writing the initial value problem in this integral form, the uniqueness of the solution will be consequence of the existence proof (Step 2 below, which uses a Banach xed point argument), the continuous dierentiability in s will be automatic from the continuity of s f s and the continuity of F (Theorem 5.15). To prove that there exists f s } s [0,1] satisfying Equation (5.3) s [0, 1], we follow the proof of Picard-Lindelöf Theorem for ODEs with some small modications. The proof is done in two steps. Lemma 5.16 (Step 1). Let 0 < t 1. If f s } s [0,t) C k+2,α ( X) satises Equation (5.3) s [0, t), then f t := (lim s t f s ) C k+2,α ( X) exists and f t N η,t (starting with f 0 N η,0 ). Proof of Step 1. If s [0, t) then s [0, t) f s = f 0 + s f s k+2,α, X f 0 k+2,α, X + f 0 k+2,α, X + ρ 0 F (f τ, τ)dτ, s 0 s 0 F (f τ, τ) k+2,α, X dτ f τ k+2,α, X dτ hence f s k+2,α, X e ρs f 0 k+2,α, X, s [0, t). In particular, for 0 s 1 s 2 < t f s2 f s1 k+2,α, X s2 s 1 F (f τ, τ) k+2,α, X dτ ρe ρt f 0 k+2,α, X s 2 s 1, i.e., f s } s [0,t) is a Cauchy limit as s t. Since C k+2,α ( X) is complete, f t := (lim s t f s ) C k+2,α ( X) exists. The inequality for f s2 f s1 k+2,α, X also implies f s } s [0,t] continuous on s [0, t], hence continuously dierentiable in s, and since f 0 N η,0, Theorem 3.4 implies f s N η,s, s [0, t]. Lemma 5.17 (Step 2). If f N η,t then there exists ɛ > 0 and a unique f s } s [t,t+ɛ) C ( [t, t + ɛ); C k+2,α ( X) ) such that f s = f + s t F (f τ, τ)dτ, s [t, t + ɛ).

21 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 21 Proof of Step 2. Let us recall that f N η,t C k+2,α ( X) if and only if the solution u of the equation (γ t u) = 0 in X u = f in X satises u(ˆx) > η. The smooth dependence of u in terms of the boundary condition and the equation coecient (Theorem 5.1), implies the existence of ɛ > 0 and δ > 0 such that if h C k+2,α ( X) satises h k+α, X < δ, then (f + h) N η,s, s [t, t + ɛ). Let us consider the following non-empty closed set of C ( [t, t + ɛ); C k+2,α ( X) ) F = f s } s [t,t+ɛ) C ( [t, t + ɛ); C k+2,α ( X) ) : f t = f, sup s [t,t+ɛ) f s f k+2,α, X < δ}. If ɛ > 0 is small enough, we can dene the following operator T : F F T (f s }) σ := T (f s } s [t,t+ɛ) ) σ := f + σ t F (f τ, τ)dτ, σ [t, t + ɛ). Let us verify that T (f s }) σ } F for f s } F. First T (f s }) t = f, also and for t σ r < t + ɛ T (f s }) σ f k+2,α, X σ T (f s }) σ T (f s }) r k+2,α, X t F (f τ, τ) k+2,α, X dτ ɛρ sup f τ k+2,α, X τ [t,σ] < ɛρ( f k+2,α, X + δ) < δ (if 0 < ɛ small enough), σ r F (f τ, τ) k+2,α, X dτ < σ r ρ( f k+2,α, X + δ) and hence σ T (f s }) σ C k+2,α ( X) is continuous and T (f s }) σ } F. In addition, if f s }, g s } F, then for any σ [t, t + ɛ) T (f s }) σ T (g s }) σ k+2,α, X σ t ɛl F (f τ, τ) F (g τ, τ) k+2,α, X dτ sup s [t,t+ɛ) [(1 + f s k+2,α, X + g s k+2,α, X ) f s g s k+2,α, X ] ɛκ(1 + 2 f k+2,α, X + 2δ)) 1 2 sup s [t,t+ɛ) f s g s k+2,α, X sup f s g s k+2,α, X (if 0 < ɛ small enough). s [t,t+ɛ)

22 22 G. BAL 1 AND M. COURDURIER 2 To go from the second to the third line we used Theorem 5.15 and the fact that f s, g s N η,s for all s [t, t + ɛ). Hence, we have that T : F F is a contraction in a closed subset of a Banach space. The Banach xed point Theorem implies the existence of a unique xed point, hence the proof of Step 2 is complete. Putting together Step 1 and Step 2, since [0, 1] is connected, we conclude the existence of a unique f s } s [0,1] that solves Equation (5.3) for all s [0, 1] (starting with f 0 N η,0 for some η > 0). This family is continuous in s, therefore continuously dierentiable in s. Completing the proof of Theorem Extensions The evolution scheme presented in the previous sections solves constructively the following problem: given a smooth enough bounded domain X and coecient γ, and given any point ˆx X, nd a boundary condition ˆf such that the solution u of (γ u) = 0 in X u = ˆf in X satises u(ˆx) 1. We now consider two possible extensions, one that imposes a condition over nitely many points instead of only one, and one that imposes a condition involving nitely many equations Finitely Many Points. Given a bounded domain X, a coecient γ and nitely many dierent points ˆx i } i I X, the goal is to nd a boundary condition ˆf, such that the solution u of the equation (γ u) = 0 in X u = ˆf in X satises u(ˆx i ) 1, i I. The process is analogous to the case of one point. We are now considering multiple constraints to be satised although we have only one equation and one boundary condition to control. The scheme proposes to start with an appropriate γ 0, f 0 (e.g. γ 0 1 and f 0 (x 1, x 2,..., x n ) = x 1 ) and construct f s such that the solution u s of (γ s u s ) = 0 in X u s = f s in X and the solution u s of (γ s u s) + (γ s u s ) = 0 u s = f s in X in X

23 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 23 satisfy d ds u s(ˆx i ) 2 = 2 u s (ˆx i ) u s(ˆx i ) 0, i I. Again, we construct f s as the solution of an initial value problem fs s = F (f s, s) f s s=0 = f 0 for an appropriate F dened below. We construct the functional F : C k+2,α ( X) [0, 1] C k+2,α ( X) as follows. We assume X is a C k+3,α bounded domain. Let γ 0, γ C k+n+3 (X) and let γ s = [(1 s)γ 0 + sγ], s [0, 1]. Assume 0 < c < γ 0, γ < C <. For s [0, 1] and for f C k+2,α ( X) let u be the C k+2,α (X) solution of (γ s u) = 0 in X u = f Let v be the C k+2,α (X) solution of (γ s v) + ((γ γ 0 ) u) = 0 v = g in X in X where g C k+2,α ( X) will be prescribed below. The dierence with the previous process appears in that we need to consider many auxiliary problems. Let λ i, i I, be the solutions of (γ s λ i ) = u(ˆx i ) δˆxi in X λ i = 0 From Theorem 3.2, λ i L p (X) and (γ s i / ν) C k+2,α ( X) depend continuously on s. Since the ˆx i } i I are dierent, the λ i } i I are linearly independent and the γ s i / ν} i I are linearly independent (as long as u(ˆx i ) 0, i I). This is proved exactly as in Theorem 3.5. By integration by parts, we obtain for all i I that u(ˆx i ) v(ˆx i ) = X λ i ((γ γ 0 ) u) X γ s i ν g, and we let g span(γ s i / ν} i I ) C k+2,α ( X) be such that, i I i γ s ν g = λ i ((γ γ 0 ) u). X X Hence, u(ˆx i ) v(ˆx i ) = 0, i I. Since the γ s i / ν} i I are linearly independent in L 2 ( X), such a g exists and is unique (as long as u(ˆx i ) 0, i I). Also, by an extension of the nite dimensional argument leading to Corollary 3.6 (adding the linear independence of the γ s i / ν} i I ), there exist constants

24 24 G. BAL 1 AND M. COURDURIER 2 C 1, C 2, independent of s and f, such that g C u 2,X 1 C f k+2,α, X 2. k+2,α, X By dening F (f, s) := g, the continuity and boundedness of F : C k+2,α ( X) [0, 1] C k+2,α ( X) can be proven exactly as it was done in the previous case (observing that the evolution with F keeps u s (ˆx i ) 1, i I). This provides the following result: Theorem 6.1. Assume X is a C k+3,α bounded domain. Let γ 0, γ C k+n+3 (X) and let γ s = [(1 s)γ 0 + sγ], s [0, 1]. Assume 0 < c < γ 0, γ < C <. Let f 0, γ 0 be chosen appropriately (e.g. γ 0 1 and f 0 (x 1, x 2,..., x n ) = x 1 ). Dene F : C k+2,α ( X) [0, 1] C k+2,α ( X) as above. Then there exists a unique solution f s } s [0,1] in C 1 ([0, 1]; C k+2,α ( X)) of the initial value problem f s s = F (f s, s) f s s=0 = f 0. The family f s } s [0,1] constructed in this way, satises that each u s, solution of (P s ) with boundary condition f s, is such that u s (ˆx i ) 1, i I, s [0, 1]. Hence ˆf = f s s=1 solves the problem presented at the beginning of this Subsection, with a condition imposed over nitely many points System of Equations. Given a bounded domain X R 3, a coecient γ and xed point ˆx X, the objective is to nd boundary conditions ˆf i } i=1,2,3, such that the solutions u i, i = 1, 2, 3 of the equations (γ u i ) = 0 in X u i (x) = ˆf i (x) in X satisfy det[( u i (ˆx)) i=1,2,3 ] 1. We now have only one constraint to satisfy. It involves multiple equations and multiple boundary conditions. The scheme proposes to start with appropriate γ 0, f0} i i=1,2,3 (e.g. γ 0 1 and f0(x i 1, x 2, x 3 ) = x i ) and construct fs, i i = 1, 2, 3 such that the solutions u i s of (γ s u i s) = 0 in X u i s(x) = fs(x) i in X and the solution (u i s) of (γ s (u i s) ) + (γ s u i s) = 0 in X (u i s) (x) = (fs) i (x) in X

25 BOUNDARY CONTROL OF ELLIPTIC SOLUTIONS 25 satisfy d ds det[( ui s(ˆx)) i=1,2,3 ] = ( ) i=1,2,3 (ui s) (ˆx) u i+1 s (ˆx) u i+2 s (ˆx) 0 (we consider the expressions i + 1 and i + 2 modulo 3). We construct fs i as the solutions of a system of ODE f s i = F i ((f i s s) i=1,2,3, s) fs i s=0 = f0 i for an appropriate (F i ) i=1,2,3 dened below. We construct the functionals F i : (C k+2,α ( X)) 3 [0, 1] C k+2,α ( X) as follows. Assume X is a C k+3,α bounded domain. Let γ 0, γ C k+n+3 (X) and let γ s = [(1 s)γ 0 + sγ], s [0, 1]. Assume 0 < c < γ 0, γ < C <. For s [0, 1] and for (f i ) i=1,2,3 (C k+2,α ( X)) 3 let u i be the C k+2,α (X) solutions of (γ s u i ) = 0 in X u i (x) = f i (x) Let v i be the C k+2,α (X) solutions of (γ s v i ) + ((γ γ 0 ) u i ) = 0 v i (x) = g i (x) in X in X, where g i C k+2,α ( X) will be prescribed below. For this system let us consider the following auxiliary problems. Let λ i, be the solutions of ( ) (γ s λ i ) = u i+1 (ˆx) u i+2 (ˆx) δˆx in X λ = 0 From Theorem 3.2, λ i L p (X) and (γ s i / ν) C k+2,α ( X) depend continuously on s. By integration by parts and summation we obtain ( ) u i+1 (ˆx) u i+2 (ˆx) v i (ˆx) = λ i ((γ γ 0 ) u i ) i=1,2,3 i=1,2,3 i=1,2,3 Let g i = µγ s i / ν, with µ chosen as µ = / λ i ((γ γ 0 ) u i ) i=1,2,3 Hence i=1,2,3 vi (ˆx) X ( ) u i+1 (ˆx) u i+2 (ˆx) i=1,2,3 = 0. X X X γ s i ν gi. ( γ s i ν By dening F i ((f i ) i=1,2,3, s) := g i we can prove the boundedness and continuity of F i : (C k+2,α ( X)) 3 [0, 1] C k+2,α ( X) as before. This yields a Theorem ) 2.

26 26 G. BAL 1 AND M. COURDURIER 2 analogous to Theorems 3.7 and 6.1. An important aspect for the argument to work is that we start with det[( u i (ˆx)) i=1,2,3 ] 1 and the evolution with F i maintains that property, hence ( ) 2 i=1,2,3 γ i X s ν remains uniformly bounded away from zero. Theorem 6.2. Assume X is a C k+3,α bounded domain. Let γ 0, γ C k+n+3 (X) and let γ s = [(1 s)γ 0 + sγ], s [0, 1]. Assume 0 < c < γ 0, γ < C <. Let (f i 0) i=1,2,3 and γ 0 be chosen appropriately (e.g. γ 0 1 and f i 0(x 1, x 2, x 3 ) = x i ). Dene F i : C k+2,α ( X) [0, 1] C k+2,α ( X) as above. Then, there exists a unique solution s (f i s) in C 1 ([0, 1]; (C k+2,α ( X)) 3 ) of the system of ODE s f i s = F i ((f i s) i=1,2,3, s) f i s s=0 = f i 0. For all s [0, 1], the solutions u i s of (P s) with corresponding boundary conditions f i s are such that det[( ui (ˆx)) i=1,2,3 ] 1. This Theorem produces ˆf = f s s=1 as the solution of the problem described at the beginning of this Subsection, with a condition involving nitely many equations. Remark 6.3. In the denition of F i above, we could redene µ = min(0, µ), resembling more closely the construction presented in Denition 3.3. Such a redenition of µ provides a boundary condition f i s} with (f i s) } of minimal L 2 ( X) 3 norm for each s [0, 1], among all f i s} that produce non-decreasing determinants. Remark 6.4. The construction presented in this Subsection works in more general settings. We may consider X R n and replace det[( u i (ˆx)) i=1,2,3 ] by L[( u i (ˆx)) i=1,...,m ] for any multi-linear function L : (R n ) m R. And if H : (R n ) m R is a continuously dierentiable function with dierential DH(z) uniformly bounded away from zero in the set z (R n ) m : H(z) 1}, then the construction presented in this Subsection also works when we replace det[( u i (ˆx)) i=1,2,3 ] by H( u i (ˆx) i=1,...,m ). Remark 6.5. Extensions for conditions involving multiple equations at nitely many points can also be addressed with this scheme. Acknowledgment GB was partially funded by NSF grant DMS and AFOSR Grant NSSEFF- FA MC was partially funded by Conicyt-Chile grant Fondecyt #

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