# THE FORM SUM AND THE FRIEDRICHS EXTENSION OF SCHRÖDINGER-TYPE OPERATORS ON RIEMANNIAN MANIFOLDS

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1 THE FORM SUM AND THE FRIEDRICHS EXTENSION OF SCHRÖDINGER-TYPE OPERATORS ON RIEMANNIAN MANIFOLDS OGNJEN MILATOVIC Abstract. We consider H V = M +V, where (M, g) is a Riemannian manifold (not necessarily complete), and M is the scalar Laplacian on M. We assume that V = V 0 + V 1, where V 0 L 2 loc(m) and C V 1 L 1 loc(m) (C is a constant) are real-valued, and M + V 0 is semibounded below on Cc (M). Let T 0 be the Friedrichs extension of ( M + V 0) C c (M). We prove that the form sum T 0 +V 1, coincides with the self-adjoint operator T F associated to the closure of the restriction to Cc (M) Cc (M) of the sum of two closed quadratic forms of T 0 and V 1. This is an extension of a result of Cycon. The proof adopts the scheme of Cycon, but it requires the use of a more general version of Kato s inequality for operators on Riemannian manifolds. 1. Introduction and the main result Let (M, g) be a Riemannian manifold (i.e. M is a C -manifold, (g jk ) is a Riemannian metric on M), dim M = n. We will assume that M is connected. We will also assume that we are given a positive smooth measure dµ, i.e. in any local coordinates x 1, x 2,..., x n there exists a strictly positive C -density ρ(x) such that dµ = ρ(x)dx 1 dx 2... dx n. We do not assume that (M, g) is complete. We will consider a Schrödinger type operator of the form H V = M + V. Here M := d d, where d: C (M) Ω 1 (M), and V L 1 loc (M) is real-valued Maximal operator. We define the maximal operator H V,max associated to H V as an operator in L 2 (M) given by H V,max u = H V u with domain Dom(H V,max ) = {u L 2 (M) : V u L 1 loc (M), H V u L 2 (M)}. (1.1) Here M u in H V u = M u + V u is understood in distributional sense. We make the following assumptions on V. Assumption A. Assume V = V 0 + V 1, where (i) V 0 L 2 loc (M) and M + V 0 is semibounded below on Cc (ii) V 1 L 1 loc (M) and V 1 C, where C > 0 is a constant. (M) Mathematics Subject Classification. Primary 35P05, 58G25; Secondary 47B25, 81Q10. 1

3 Remark 2.3. In case of the operator + V 0 in an open set M R n, Theorem 2.2 was proven in [2, Th. 1]. We will first prove the following special case of Theorem 2.2 Proposition 2.4. Suppose that C V 0 L 2 loc (M), where C > 0 is a constant. Let T b be the Friedrichs extension of ( M + V 0 ) C c (M). Then T b has a positive form core. We begin with a few preliminary lemmas. In what follows T b is as in the hypothesis of Proposition 2.4, and t b is the closed quadratic form associated with T b. Without the loss of generality, we may and we will assume that V 0 0 so that T b is a positive self-adjoint operator. We will denote W 1,2 (M) := {u L 2 (M): du L 2 (T M)}. By W 1,2 0 (M) we will denote the closure of Cc (M) in the norm u 2 W := du 2 + u 2, where is the L 2 norm. By Q(V 1,2 0 ) we will denote the set {u L 2 (M): V 1/2 0 u L 2 (M)}. Clearly, Q(V 0 ) is the closure of Cc (M) in the norm u 2 V 0 := V 1/2 0 u 2 + u 2, (2.1) where is the norm in L 2 (M). In proofs of the following three lemmas, we will use the arguments from the proof of Lemma 1 in [5]. Lemma 2.5. Q(T b ) = W 1,2 0 (M) Q(V 0 ). Proof. Denote by H 1 := W 1,2 0 (M) Q(V 0 ). Consider a sesquilinear form S : H 1 H 1 C given by S(u, v) := (du, dv) + (V 1/2 0 u, V 1/2 0 v), where (, ) is the inner product in L 2. This sesquilinear form is closed, so the pre-hilbert space H 1 is complete in the norm (u, v) tb := (du, dv) + (V 1/2 0 u, V 1/2 0 v) + (u, v). (2.2) By definition of W 1,2 0 (M) and Q(V 0 ), it follows that H 1 is the closure of Cc (M) in the norm tb corresponding to (2.2). For all u, v Cc (M), (u, v) tb = (u, v)+(t b u, v). By Theorem 14.1 in [3], Q(T b ) is the closure of Cc (M) in the norm tb corresponding to (2.2), so Q(T b ) = W 1,2 0 (M) Q(V 0 ). Lemma 2.6. Assume that u C c (M). Then there exists a sequence φ k C c (M) + such that φ k u tb 0 as k, where tb is the norm corresponding to (2.2). Proof. Let u Cc (M). Then u Wcomp(M). 1,2 Using a partition of unity we may assume that u is supported in a coordinate neighborhood. Let u ρ = J ρ u, where J ρ is the Friedrichs mollifying operator, cf. Sect in [1]. Then u ρ Cc (M). It is well-known that u ρ u 3

4 as ρ 0+ both in the space Wcomp(M) 1,2 and in the space L 2 comp(m). Also, since u is continuous compactly supported on M and V 0 L 2 loc (M), we have V 0 ( u ρ ) 2 dµ V 0 u 2 dµ as ρ 0 +. (2.3) Therefore, u ρ u tb 0 as ρ 0+, (2.4) where tb is the norm corresponding to (2.2). Lemma 2.7. Suppose that u Q(T b ). Then u Q(T b ). Proof. Let u Q(T b ). By Lemma 2.5, we get u W 1,2 0 (M) Q(V 0 ). Since u W 1,2 0 (M), Lemma 7.6 from [4] gives u W 1,2 0 (M). From u Q(V 0 ), we immediately get u Q(V 0 ). Therefore, u W 1,2 0 (M) Q(V 0 ), so by Lemma 2.5, we obtain u Q(T b ) Proof of Proposition 2.4. We will follow the proof of Lemma 2 in [2]. Suppose that u Q(T b ) +. By Lemma 2.5, there exists a sequence φ j Cc (M) such that φ j u tb 0 as j, (2.5) where tb is the norm corresponding to (2.2). In what follows, we will denote (sign w)(x) := w(x) w(x) when w(x) 0, and 0 otherwise. We have φ j u 2 t b = φ j u 2 + d φ j du 2 + V 1/2 0 ( φ j u) 2 where denotes the norm L 2. From (2.6) we obtain φ j u 2 + d φ j du 2 + V 1/2 0 (φ j u) 2 = φ j u 2 + Re((sign φ j )dφ j ) du 2 + V 1/2 0 (φ j u) 2, (2.6) φ j u 2 t b φ j u 2 + [ (sign φ j )(dφ j du) + (sign φ j 1)du ] 2 + V 1/2 0 (φ j u) 2 φ j u 2 + [ dφ j du + (sign φ j 1)du ] 2 + V 1/2 0 (φ j u) 2, (2.7) where denotes the norm in L 2. By (2.5), the first, second and fourth term on the right hand side of (2.7) go to 0 as j. It remains to show that (sign φ j 1)du 0 as j. (2.8) Since φ j u in L 2 (M), a lemma of Riesz shows that there exists a subsequence φ jk such that φ jk u a.e dµ, as k. By Lemma 7.7 from [4], it follows that du = 0 almost everywhere on {x M : u(x) = 0}. Hence, as k, sign φ jk 1 a.e. on M. Since du L 2 (T M), dominated convergence theorem immediately implies (2.8) (after passing to the chosen subsequence φ jk ). 4

5 This shows that φ jk u tb 0 as k. (2.9) By (2.9) and Lemma 2.6, there exists a sequence ψ l in Cc (M) + such that ψ l u tb 0 as l. By Definition 2.1 it follows that T b has a positive form core. In what follows, we will use a version of Kato s inequality. For the proof of this inequality in general setting, cf. Theorem 5.6 in [1]. Theorem 2.9. Let E be a Hermitian vector bundle on M, and let : C (E) C (T M E) be a Hermitian connection on E. Let : C (T M E) C (E) be formal adjoint of with respect to the usual inner product on L 2 (E). Assume that u L 1 loc (E) and u L 1 loc (E). Then where M u Re u, sign u, (2.10) sign u(x) = { u(x) u(x) if u(x) 0, 0 otherwise. Definition Let (X, µ) be a measure space. A bounded linear operator A: L 2 (X, µ) L 2 (X, µ) is said to be positivity preserving if for every 0 u L 2 (X, µ) we have Au 0. We will also use the following abstract theorem due to Simon, cf. Theorem 2.1 in [11]. Theorem 2.11 (Simon [11]). Suppose that (X, µ) is a measure space. Suppose that H is a positive self-adjoint operator in L 2 (X, µ). Then (H + 1) 1 is positivity preserving if and only if the following two conditions are satisfied (i) For every u Q(H), we have u Q(H). (ii) For every u Dom(H) and 0 v Q(H), the following is true Re[h( u, v)] Re((sign u)v, Hu), where h is the quadratic form associated to H, and (sign u)(x) = u(x) u whenever u(x) 0, and 0 otherwise. The following lemma extends Lemma 1 from [5] to the case of Riemannian manifolds. Lemma The operator (T b + 1) 1 is positivity preserving. Proof. Let t b be the quadratic form associated to T b. By Theorem 2.11, it suffices to check the following conditions (i) For every u Q(T b ), we have u Q(T b ) and (ii) For every u Dom(T b ) and 0 v Q(T b ), the following is true Re[t b ( u, v)] Re((sign u)v, T b u) 5

6 Condition (i) follows immediately by Lemma 2.7. We now prove that the condition (ii) holds. Let u Dom(T b ). Then ( M + V 0 )u L 2 (M) and hence M u L 1 loc (M). For u Dom(T b ) and 0 φ Cc (M) we have, Re[t b ( u, φ)] = Re( u, ( M + V 0 )φ) = ( u, M φ) + ( u, V 0 φ) = ( M u, φ) + (V 0 u, φ) Re((sign ū) M u, φ) + ((sign ū)v 0 u, φ) = Re((sign ū)t b u, φ) = Re((sign u)φ, T b u). (2.11) Here we used integration by parts and the special case of Kato inequality (2.10) for M. Let 0 v Q(T b ). By Proposition 2.4, there exists a sequence φ j Cc (M) + such that φ j v tb 0 as j, where tb is the norm corresponding to (2.2). From (2.11), we obtain Re[t b ( u, v)] = lim j Re[t b ( u, φ j )] lim j Re((sign u)φ j, T b u) = Re((sign u)v, T b u). This proves condition (ii) and the lemma. In what follows T 0 is as in hypothesis of Theorem 2.2. Without the loss of generality we may and we will assume that T 0 is a positive self-adjoint operator. We will also use the following notation Z + := {1, 2, 3,... }. Proposition (T 0 + 1) 1 is positivity preserving. Proof. We will adopt the arguments from the proof of Lemma 2 in [5] to our setting. For every k Z + and x M, define { V0 (x) if V 0 (x) k, Q k (x) = k if V 0 (x) < k. Let T k be the Friedrichs extension of ( M + Q k ) Cc (M). Then for all k Z + and u Cc (M), we have (u, T k u) (u, T 0 u) 0, (2.12) where (, ) is the inner product in L 2 (M). From (2.12) it follows that T 0 T k for all k Z +, (2.13) i.e. Q(T k ) Q(T 0 ), and for all u Q(T k ), t 0 (u, u) t k (u, u), where t 0 and t k are the quadratic forms associated to T 0 and T k respectively. Furthermore, for all u Cc (M), the following is true (u, T k u) (u, T 0 u) as k. (2.14) Clearly, Cc (M) Q(T k ) for all k Z +. By definition of Friedrichs extension, it follows that Cc (M) is dense in Q(T 0 ) (in the norm of Q(T 0 )). This, (2.13) and (2.14) show that the hypotheses of abstract Theorem 7.9 from [3] are satisfied. Therefore, as k, T k T 0 in the strong resolvent sense. 6

7 By Lemma 2.12, (T k + 1) 1 is positivity preserving for all k Z +. Therefore, (T 0 + 1) 1 is also positivity preserving. Corollary Assume that u Q(T 0 ). Then u Q(T 0 ). Proof. T 0 is a positive self-adjoint operator in L 2 (M). By Proposition 2.13, the operator (T 0 + 1) 1 is positivity preserving. Now the corollary follows immediately from Theorem Truncation operators corresponding to T 0. Let T 0 be as in hypothesis of Theorem 2.2. Define V 0 + := max{v 0, 0}, V0 := max{ V 0, 0}, and for each k Z +, let V0 k := min{k, V0 }. Denote by T + and T k the Friedrichs extension of ( M +V 0 + ) Cc (M) and ( M +V 0 + V 0 k) Cc (M) respectively. Let t 0, t + and t k (k Z + ) be the quadratic forms associated to T 0, T + and T k respectively. The following lemma is analogous to Lemma 3 in [2]. Lemma With the notations of Sect. 2.15, (i) T k T 0 in the strong resolvent sense as k. (ii) Q(T + ) Q(T 0 ). Proof. For all k Z +, we clearly have T 0 T k. Also, Cc (M) Q(T k ) for all k Z +. By definition of T 0 it follows that Cc (M) is dense in Q(T 0 ) (in the norm of Q(T 0 )). Clearly, for all w Cc (M) (w, T k w) (w, T 0 w) as k. We now apply Theorem 7.9 in [3] to conclude the proof of (i). Property (ii) follows immediately since T + T Proof of Theorem 2.2. We will adopt the arguments from the proof of Theorem 1 in [2]. By the proof of Lemma 2.16 it follows that and Q(T + ) Q(T k ) Q(T 0 ) (2.15) t0 tk t+, (2.16) where t0, tk and t+ are the norms associated to t 0, t k and t + respectively, cf. (1.2). In fact, Q(T k ) = Q(T + ) since the norms tk and t+ are equivalent, because V 0 + V 0 k and V + 0 differ by a bounded function. By Proposition 2.4, T + has a positive form core, i.e. for every u Q(T + ) +, there exists a sequence φ j Cc (M) + such that φ j u t+ 0 as j. By (2.16) it follows that φ j u t0 0 as j. To prove the theorem, it remains to show that for every w Q(T 0 ) +, there exists a sequence w j Q(T + ) + such that w j w t0 0 as j. (2.17) 7

8 Let w Q(T 0 ) +. For every k, l Z + define ( ) 1 1 w l := l T w and ( ) 1 1 wl k := l T k + 1 w. This makes sense since 0 T 0 T k are self-adjoint operators. By Lemma 2.12, the operator (T k + 1) 1 is positivity preserving. Hence wl k Dom(T k ) + Q(T k ) + = Q(T + ) +. Since the operators (T 0 + 1) 1/2 and (T 0 /l + 1) 1 commute, we have ( (1 ) 1 w l w t0 = l T ) (T 0 + 1) 1/2 w, (2.18) where denotes L 2 (M) norm. Clearly, ( ) 1 1 l T strongly as l. This and (2.18) show that w l w t0 0 as l. (2.19) Fix l Z +. For each k Z +, let t 0 + l and t k + l denote the quadratic forms corresponding to (positive self-adjoint) operators T 0 + l and T k + l respectively. Let t0 +l and tk +l denote the norms in Q(T 0 + l) and Q(T k + l) respectively, cf. (1.2). The corresponding inner products will be denoted by (, ) t0 +l and (, ) tk +l. Using (2.15), (2.16) and Cauchy-Schwarz inequality we have for all w Q(T 0 ) + (T k + l) 1 w (T 0 + l) 1 w 2 t 0 +l = (T k + l) 1 w 2 t 0 +l + (T 0 + l) 1 w 2 t 0 +l 2((T k + l) 1 w, (T 0 + l) 1 w) t0 +l (T k + l) 1 w 2 t k +l + (T 0 + l) 1 w 2 t 0 +l 2((T k + l) 1 w, (T 0 + l) 1 w) t0 +l = ((T 0 + l) 1 w, w) ((T k + l) 1 w, w) + (1 l)[ (T k + l) 1 w 2 + (T 0 + l) 1 w 2 2((T k + l) 1 w, (T 0 + l) 1 w)] ((T 0 + l) 1 w, w) ((T k + l) 1 w, w) (T 0 + l) 1 w (T k + l) 1 w w, (2.20) where (, ) is the inner product in L 2 (M) and is the norm in L 2 (M). By Lemma 2.16, it follows that for fixed l Z +, T k + l T 0 + l in the strong resolvent sense as k. Clearly, for any positive self-adjoint operator A, (A/l+1) 1 = l(a+l) 1. Therefore by (2.20), for a fixed l Z + w k l w l t0 +l 0 as k. 8

9 This is equivalent to w k l w l t0 0 as k. (2.21) Since wl k Q(T + ) +, we can use (2.19) and (2.21) to choose a subsequence {w j } from {wl k} so that (2.17) holds. This concludes the proof of the theorem. 3. Proof of Theorem 1.5 We essentially follow the proof of Theorem 2 in [2]; however, we need to use Kato inequality (2.10) for operators on manifolds. Without the loss of generality, we may and we will assume that M + V 0 0 and V 1 0. Let us denote T q := T 0 +V 1 and let t min and t q be as in Sect. 1.4 and Sect Since t min and t q coincide on C c (M), it is sufficient to show that C c (M) is dense in the Hilbert space Q(T q ) = Q(T 0 ) Q(V 1 ) with the inner product (, ) tq := t q (, ) + (, ) L 2 (M), where t q (, ) is the sesquilinear form obtained by polarization of t q. Let v Q(T q ) be orthogonal to C c (M) in (, ) tq. This means that for all w C c (M), (( M + V 0 + V 1 )v, w) L 2 (M) + (v, w) L 2 (M) = 0. This leads to the following distributional equality Since V 1 L 1 loc (M) and v Q(V 1), we have M v = (V 0 + V 1 + 1)v. (3.1) 2 V 1 v = 2 V 1 v V 1 + V 1 v 2 which immediately gives V 1 v L 1 loc (M). Since V 0 L 2 loc (M), it follows that V 0v L 1 loc (M). From (3.1) we obtain Mv L 1 loc (M). Using Kato inequality (2.10) in case = d and the equation (3.1), we get M v Re(sign v M v) = V 0 v V 1 v v (V 0 + 1) v. (3.2) The last inequality in (3.2) holds since V 1 0. From (3.2), we obtain the following distributional inequality ( M + V 0 + 1) v 0. (3.3) Let T 0 be as in hypothesis, and let t 0 denote the closed quadratic form associated to T 0. Using (3.3), we get ((T 0 + 1)w, v ) L 2 (M) 0 for all w C c (M) +. (3.4) Since v Q(T 0 ), Corollary 2.14 gives v Q(T 0 ). Therefore, we can write (3.4) as (w, v ) t0 0 for all w C c (M) +, (3.5) where (, ) t0 = t 0 (, ) + (, ) L 2 (M) denotes the inner product in Q(T 0 ). 9

10 Let f := (T 0 + 1) 1 v. By Proposition 2.13, (T 0 + 1) 1 is positivity preserving, so f Dom(T 0 ) + Q(T 0 ) +. By Theorem 2.2, T 0 has a positive form core. Therefore, there exists a sequence f k Cc (M) + such that lim (f k, v ) t0 = (f, v ) t0 = ((T 0 + 1) 1 v, v ) t0 = v 2, (3.6) k where v and (, ) t0 are as in (3.5), and is the norm in L 2 (M). From (3.5) and (3.6) we obtain v 2 0, i.e. v = 0. This shows that Cc (M) is dense in Q(T q ), and the theorem is proven. References [1] M. Braverman, O. Milatovic, M. A. Shubin, Essential self-adjointness of Schrödinger type operators on manifolds, Preprint math.sp/ , 23 January [2] H. L. Cycon, On the form sum and the Friedrichs extension of Schrödinger operators with singular potentials, J. Operator Theory, 6 (1981), [3] W. G. Faris, Self-adjoint Operators, Lecture Notes in Mathematics No. 433, Springer-Verlag, Berlin e.a., [4] D. Gilbarg, N. S. Trudinger, Elliptic partial differential equations of second order, Springer, New York, [5] H.-W. Goelden, On non-degeneracy of the ground state of Schrödinger operators, Math. Z., 155 (1977), [6] H. Hess, R. Schrader, D.A. Uhlenbrock, Domination of semigroups and generalization of Kato s inequality, Duke Math. J., 44 (1977), [7] H. Hess, R. Schrader, D. A. Uhlenbrock, Kato s inequality and the spectral distribution of Laplacians on compact Riemannian manifold, J. Differential Geom., 15 (1980), [8] T. Kato, Perturbation theory for linear operators, Springer-Verlag, New York, [9] T. Kato, A second look at the essential selfadjointness of the Schrödinger operators, Physical reality and mathematical description, Reidel, Dordrecht, 1974, [10] M. Reed, B. Simon, Methods of Modern Mathematical Physics I, II: Functional analysis. Fourier analysis, self-adjointness, Academic Press, New York e.a., [11] B. Simon, An abstract Kato s inequality for generators of positivity preserving semigroups, Indiana Univ. Mat. J., 26 (1977), [12] B. Simon, Kato s inequality and the comparison of semigroups, J. Funct. Anal., 32 (1979), [13] B. Simon, Maximal and minimal Schrödinger forms, J. Operator Theory, 1 (1979), [14] M. Taylor, Partial Differential Equations II: Qualitative Studies of Linear Equations, Springer-Verlag, New York e.a., Department of Mathematics, Northeastern University, Boston, MA 02115, USA address: 10

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