GRADIENT ESTIMATES FOR THE GROUND STATE ¨ SCHRODINGER EIGENFUNCTION AND APPLICATIONS ˜ ¨ RODRIGO BANUELOS AND PAWEL KROGER

1. Introduction Let Ω be a bounded convex domain in Euclidean space Rn . Consider the Schr¨odinger operator −4 + V for a nonnegative convex potential V on Ω under Dirichlet boundary conditions. Under these assumptions the eigenvalues are discrete and satisfy 0 < λV1,Ω < λV2,Ω ≤ λV3,Ω . . . . When the potential is identically zero we will just write λi,Ω for these eigenvalues. The quantity λV2,Ω − λV1,Ω is called the spectral gap. It was conjectured by M. van den Berg [4] that λV2,Ω − λV1,Ω can be estimated below by 3π 2 /d2Ω where dΩ denotes the diameter of Ω. (See also [1], [2] and Problem 44 in [14].) The lower bound π 2 /4d2Ω was obtained in [12]. This bound was subsequently improvement to π 2 /d2Ω in [11] (see also [9]). For V = 0 and planar convex domains which are symmetric in both axes, Smits [13] obtained a similar lower bound with the diameter replaced by the length of the longest axes of symmetry. The symmetry assumption is needed in order to apply a result by L. Payne [10] which guarantees the existence of an eigenfunction for the second eigenvalue whose nodal line is one of the axes of symmetry. The conjectured lower bound was proved in [6] for the Laplacian (the case V = 0) in planar domains which are again symmetric in both axes. A different proof with some extensions is given in [3]. The purpose of this note is to give a simple proof for a first order differential inequality that implies the gap estimate when V = 0 under the symmetry assumptions on Ω. Our proof is based on the maximum principle technique. As in the case of the results in [3] and [6], the full convexity of the domain is not needed if the domain is symmetric and convex relative both coordinate axes. 2. The result n

Let Ω ⊂ R be a convex domain which can be included in a strip (infinite slab) S of width 2b. We may assume that that strip is perpendicular to the first coordinate axis and centered with respect to the origin. That is, S = {x = (x1 , x2 , ..., xn ) | |x1 | ≤ b}. We aim to compare the ground state eigenfunction of the Schr¨ odinger operator −4 + V on Ω ⊂ Rn with the ground state eigenfunction of −4 in S. We will assume that both V and Ω are symmetric with respect to the hyperplane x1 = 0 and that V is increasing in x1 for x1 ≥ 0. Under these assumptions we have

The first author was supported in part by NSF grant # 9700585-DMS . The second author was supported in part by Fondecyt Grant # 1000713 and by UTFSM Grant # 120023. 1

˜ ¨ RODRIGO BANUELOS AND PAWEL KROGER

2

Proposition 1. Let φ1 denote the ground state eigenfunction of −4 + V on Ω and let ψ1 denote the ground state eigenfunction of −4 on S. Then ∂ ∂ ln(φ1 )(x) < ln(ψ1 )(x), ∂x1 ∂x1 for every x ∈ Ω with x1 > 0. Proof. We can and will assume without loss of generality that the potential V ∂ V (x) > 0 for every x with x1 > 0, that Ω has a smooth is smooth and that ∂x 1 boundary and that the coordinate axis x1 is not tangent to that boundary for x1 > 0, and that the closure of Ω is contained in the interior of the strip S. The general case follows from a limit argument. Moreover, we will replace the strip by a translate S = {(x1 , x2 , ..., xn ) | |x1 + | ≤ b} for a positive  such that the closure of Ω is still contained in the interior of S . By symmetry, ∂ ∂ () (ln φ1 )(x) = 0 < (ln ψ1 )(x) ∂x1 ∂x1 for every x ∈ Ω with x1 = 0. On the other hand, by Hopf’s boundary lemma ∂ ln(φ1 )(x) = −∞ ∂x1 for every x ∈ ∂Ω with x1 > 0. Thus we only have to show that the assertion is correct in the interior of Ω+ ≡ Ω ∪ {(x1 , x2 , ..., xn ) | x1 > 0}. Assume that ∂ ∂ () (ln φ1 )(x) − (ln ψ1 )(x) ∂x1 ∂x1 attains a nonnegative maximum at an interior point zM of Ω+ . We emphasize that it is sufficient to show the assumption that that maximum is equal to 0 leads to a contradiction (cf. the last section of [12] for the method of continuity). Indeed, if the maximum is strictly positive, then we can and will replace the strip S by a strip with width greater than 2b in order to make the above maximum equal to 0 (take into account that ln(φ1 ) is concave [5] and [12]). Thus, ∇x

∂ ∂ () (ln φ1 )(zM ) = ∇x (ln ψ1 )(zM ) ∂x1 ∂x1

4x

∂ ∂ () (ln φ1 )(zM ) ≤ 4x (ln ψ1 )(zM ). ∂x1 ∂x1

and (1)

()

∂ Obviously, ∇x ∂x (ln ψ1 )(zM ) is parallel to the x1 -axis. Recall also that 1

∂ ∂ () (ln φ1 )(zM ) = (ln ψ1 )(zM ). ∂x1 ∂x1 Thus, < ∇x

∂ (ln φ1 )(zM ) , ∇x (ln φ1 )(zM ) > ∂x1

∂ () () (ln ψ1 )(zM ) , ∇x (ln ψ1 )(zM ) >, ∂x1 where < , > denotes the inner product. On the other hand, differentiating (2)

=< ∇x

4x (ln φ1 ) + |∇x ln φ1 |2 − V + λV1,Ω = 0

GRADIENT ESTIMATES AND APPLICATIONS

3

in the direction of x1 , we obtain that 4x

∂ ∂ ∂ (ln φ1 ) + 2 < ∇x (ln φ1 ) , ∇x (ln φ1 ) > − V = 0. ∂x1 ∂x1 ∂x1

Similarly, 4x

∂ ∂ () () () (ln ψ1 ) + 2 < ∇x (ln ψ1 ) , ∇x (ln ψ1 ) > = 0. ∂x1 ∂x1

∂ V < 0, we have arrive at a contradiction to (1) and (2) and that comSince − ∂x 1 pletes the proof.

Theorem 1. Suppose that Ω and V are as in the above Proposition. Suppose that there is an odd eigenfunction φ2 of −4 + V with respect to x1 . That is, φ2 (−x1 , x2 , ..., xn ) = −φ2 (x1 , x2 , ..., xn ) for all (x1 , x2 , ..., xn ) ∈ Ω. Then λV2,Ω − λV1,Ω ≥ 3π 2 /4b2 ≥ 3π 2 /d2Ω . Proof. Let χ ≡ φ2 /φ1 . Then (3)

λV2,Ω − λV1,Ω

Z =

|∇x χ|2 φ21 dx /

Z



Z ≥

(4)

∂ | χ|2 φ21 dx / Ω ∂x1

χ2 φ21 dx



Z

χ2 φ21 dx.



Clearly the quantity Z

Z ∂ χ|2 φ21 dx1 / χ2 φ21 dx1 , I ∂x1 I where I is the set of all x1 such that (x1 , x2 , ..., xn ) belongs to Ω, is bounded from below by the first nonzero Neumann eigenvalue µ1 of the operator |

(5)



∂ ∂2 ∂ −2 ln φ1 ∂x21 ∂x1 ∂x1

∂χ /χ (cf. [7], the end of section 2.15). Thus, the eigenvalue µ1 is on I. Set u ≡ ∂x 1 the smallest positive number µ such that the Ricatti equation

∂ ∂ u + u2 + 2( ln φ1 )u + µ = 0 ∂x1 ∂x1 has a solution on I ∩ [0, ∞) that satisfies limx1 ↓0 u(x1 ) = +∞ and inf I∩[0,∞) u = 0. The Ricatti equation ∂ ∂ v + v 2 + 2( ln ψ1 )v + µ = 0 ∂x1 ∂x1 is obtained in a similar way. Thus we have arrived at a pair of first order equations where the second equation satisfies the standard uniqueness theorem. The above Proposition implies via pointwise comparison of the solutions of the two Riccati equations that the eigenvalue µ1 is bounded from below by the first nonzero Neumann eigenvalue of the operator (6)



∂ ∂2 ∂ −2 ln ψ1 2 ∂x1 ∂x1 ∂x1

on (−b, b), which is 3π 2 /4b2 . This completes the proof of the Theorem.

˜ ¨ RODRIGO BANUELOS AND PAWEL KROGER

4

Remark 1. Alternatively, we can use the variational characterization in order to compare the lowest eigenvalue of the operator (5) with that of (6). Let I = (−a, a) where a ≤ b. Let u be the eigenfunction of the problem (5) in I corresponding to the lowest nonzero eigenvalue µ1 . This function, in addition to u0 (a) = u0 (−a) = 0, satifies (from the symmetry of the situation) i) uu0 ≥ 0 for x ≥ 0, uu0 ≤ 0 for x ≤ 0, and ii) u(−x) = −u(x). By ii) we see that Z a u(x)ψ12 (x) dx = 0 −a

where ψ1 is the eigenfunction for the Laplacian in (−a, a). Let ν1 be the lowest nonzero eigenvalue of the problem (6) restricted to the interval (−a, a). By domain 2 monotonicity, ν1 ≥ 3π 4b2 . Testing with the eigenfuntion u of problem (5) in problem (6) in the interval (−a, a), we get that Z a Z a ν1 u2 |ψ1 |2 dx ≤ |u0 |2 |ψ1 |2 dx −a −a Z a 0 = − u u0 ψ12 dx −a Z a Z a = − uu00 ψ12 dx − 2 uu0 ψ1 ψ10 dx −a

−a

where we have used the “prime” notation to denote derivatives in the x1 direction. From (5) we know that −uu00 ψ12 = µ1 ψ12 |u|2 + 2ψ12 Hence, Z ν1

a

u2 |ψ1 |2 dx ≤ µ1

−a

Z

a

u2 ψ12 dx + 2

−a

Z

φ01 0 u u. φ1

a

u0 u

−a



ψ0 φ01 − 1 φ1 ψ1



ψ12 dx

or   0 ψ10 φ1 2 0 − u u φ1 ψ1 ψ1 dx −a 3π Ra ≤ ν1 ≤ µ1 + 2 . 4b2 u2 |ψ1 |2 dx −a 2

Ra

This together with property i) of u, the Proposition, and (3) and (4) proves the comparison result. By the result of Payne [10], for V = 0 and Ω ⊂ R2 symmetric with respect to both axes and convex on both axes, there exists an eigenfunction corresponding to λ2 whose nodal line is the intersection of the domain with one of the two axes. Therefore we have Corollary 1. Suppose V = 0 and let Ω ⊂ R2 be symmetric and convex with respect to both coordinate axes. Let l = 2b be the length of its major axis. Then λ2,Ω − λ1,Ω ≥ 3π 2 /l2 .

GRADIENT ESTIMATES AND APPLICATIONS

5

References [1] Ashbaugh, M.; Benguria, R. Optimal lower bounds for eigenvalue gaps for Sch¨ odinger operators with symmetric single well potentials and related results, Maximum principles and eigenvalue problems in partial differential equations. Longman, White Plains, NY (1988). [2] Ashbaugh, M.; Benguria, R. Optimal lower bounds for the gap between the first two eigenvalues of one–dimensional Schr¨ odinger operators with symmetric single–well potentials, Proc. Amer. Math. Soc., 105, 419–424, (1989). [3] Ba˜ nuelos, R.; M´ endez-Hern´ andez, P. J. Sharp inequalities for heat kernels of Schr¨ odinger operators and applications to spectral gaps, J. Funct. Anal. 176, 368–399, (2000). [4] van den Berg, M. On condensation in the free–boson gas and the spectrum of the laplacian, J. Statist. Phys. 31, 623–637, (1983). [5] Brascamp, H. J.; Lieb, E. H. On extensions of the Brunn-Minkowski and Pr´ ekopa-Leindler theorems, including inequalities for log concave functions, and with an application to the diffusion equation, J. Funct. Anal. 22, 366–389, (1976). [6] Davis, B. On the spectral gap of the Dirichlet Laplacian, Arkiv. Mat (to appear). [7] Ince, E. L. Ordinary differential equations, Dover Publ. 1956 [8] Kr¨ oger, P. An extension of a theorem by Brascamp and Lieb, Proc. XIV Encuentro de la Zona Sur, Lican Ray, 2000. [9] Lin, J. A lower bound for the gap between the first two eigenvalues of Schr¨ odinger operators on convex domains of S n and Rn , Michigan Math. J. 40, 259–270, (1993). [10] Payne, L. On two conjectures in the fixed membrane eigenvalue problem, Z. Angew. Math. Phys. 24 , 721–728 (1973) [11] Qihuang, Y.; Zhong, J. Q. Lower bounds for the gap between the first and second eigenvalues for the Schr¨ odinger operator, Trans Amer. Math. Soc. 294, 341–349, (1986). [12] Singer, I. M.; Wang, B.; Yau, S.-T.; Yau, St. S.-T. An estimate of the gap of the first two eigenvalues of the Schr¨ odinger operator, Ann. Scuola Norm. Sup. Pisa Cl. Sci. 12, 319-333. (1985). [13] Smits, R. Spectral gaps and rates to equilibrium for diffusions in convex domains, Michigan Math. J. 43, 141–157, (1996). [14] Yau, S.-T. Open problems in geometry, Proceedings of symposia in pure mathematics, vol. 54, 1–22, (1993). Mathematics Department, Purdue University, West Lafayette, IN 47907 E-mail address: [email protected] ´tica, UTFSM, Valpara´ıso, Chile Departamento de Matema E-mail address: [email protected]

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SCHR¨ODINGER EIGENFUNCTION AND APPLICATIONS ..... [14] Yau, S.-T. Open problems in geometry, Proceedings of symposia in pure mathematics, vol.

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