Some Topics in the Theory of Harmonic Functions on Riemannian Manifolds

Some topics in the theory of harmonic functions on Riemannian manifolds

Stefano Pigola

Bicocca, July 12, 2006

Abstract

In this talk we take a look at some classical and some new results on the interplay between the vanishing and the ﬁnite-dimensionality of spaces of special harmonic functions on one hand, and the geometry of the underlying complete Riemannian manifold on the other hand.

Contents

1 Prelude 1

2 Lp<+∞-harmonic functions 5 2.1 Yau vanishing result and Li-Schoen counter-examples ...... 5 2.2 Li-Schoen Lp-mean value inequalities ...... 6

3 (Semi-)Bounded harmonic functions 6 3.1 Parabolicity and volumes ...... 7 3.2 Structure at inﬁnity ...... 8 3.3 Negatively curved manifolds ...... 9 3.4 Non-negatively curved manifolds ...... 10

4 Harmonic functions with finite Lp-energy 10 4.1 An example in negative curvature ...... 11 4.2 Spectral assumptions: vanishing and finiteness results ...... 11 4.3 Structure at infinity ...... 13

1 Prelude

In what follows (M, h, i) always denote a connected, geodesically complete Riemannian manifold (without boundary) of dimension m ≥ 2. Its Levi-Civita connection is denoted by D. Moreover, we set

Riem (X,Y ) Z = DX DY Z − DY DX Z − D[X,Y ]Z for its Riemann curvature tensor and

hRiem (X,Y ) Y,Xi Sec (X ∧ Y ) = |X|2 |Y |2 − hX,Y i2

1 for the sectional curvature along the 2-plane Π spanned by the linearly independent vector ﬁelds X,Y . The Ricci curvature of M is denoted by

Ric (Y,Z) = traceh,i (X → Riem (X,Y ) Z) m X = hRiem (Ej,Y ) Z,Eji j=1 where {Ei} is any o.n. frame. The Riemannian metric h, i of M gives rise to a natural intrinsic distance dist and a natural Riemannian measure dvol which give M the structure of a metric measure space. The distance is deﬁned by dist (x, y) = inf length (γ) , γ the inﬁnmum being taken with respect to any piece-wise smooth curve γ from x to y. Since (M, h, i) is geodesically complete, according to the Hopf-Rinow Theorem, (M, dist) is a complete metric space with the Heine-Borel property. Thus, setting

BR (o) = {x ∈ M : dist (o, x) < R} and

∂BR (o) = {x ∈ M : dist (o, x) = R} for the metric ball and sphere centered at o ∈ M and of radius R > 0, we have that both BR (o) and ∂BR (0) are compact subsets of M. The volume of balls vol (BR) and the area of spheres vol (∂BR) can be computed by means of the Riemannian measure that, in local coordinates x1, ..., xm, writes as √ dvol = Gdx1...dxm where

G = det (h∂i, ∂ji) .

Now, let u : M → R be a given function. Recall that the gradient of u is the vector field ∇u on M defined by h∇u, Xi = X (u) for any vector field X on M. Said differently, ∇u is the h, i-dual of the differential du. The Hessian of u is the symmetric 2-tensor defined by

Hess (u)(X,Y ) = hDX ∇u, Y i .

Finally, the Laplacian of u is the divergence-form, linear operator given by

∆u = traceh,iHess (u) m X = Hess (u) DEj ∇u, Ej j=1 = div (∇u) .

2 Direct computations show that, in local coordinates x1, ..., xm,

ki ∇u = g ∂ku ∂i √ 1 kl Hess (u)(∂i, ∂j) = √ gjk∂i Gg ∂lu G √ 1 jl ∆u = √ ∂j Gg ∂lu , G where

gij = h∂i, ∂ji

G = det (gij) ij −1 g = (gij) .

In particular jl ∆u = g ∂j∂lu + ﬁrst-order terms is a second order, elliptic operator. For those who prefer the moving frame notation, having ﬁxed a local o.n. coframe θj , we have,

i du = uiθ i j j k Hess (u) = uijθ ⊗ θ , uijθ = dui − ukθi X ∆u = ujj. j

n i o Here, θj are the connection forms deﬁned by the ﬁrst Cartan equations

( i i j dθ = −θj ∧ θ i j θj + θi = 0.

Thus, the function u : M → R is said to be harmonic if it satisﬁes

∆u = 0.

When the equality is replaced by the inequality

∆u ≤ 0 we say that u is superharmonic. Finally reversing the inequality

∆u ≥ 0 we get a subharmonic function. Easy application of the strong maximum principle (alternatively you can apply the divergence theorem) shows that:

Fact 1 Let (M, h, i) be compact. Then every harmonic (subharmonic, superharmonic) function is constant.

3 A question arises naturally. Let (M, h, i) be a generic non-compact manifold; does M support any non-constant harmonic function? The next result answers the question in the aﬃrmative by showing that, in fact, there are “many” harmonic functions on M.

Theorem 2 (Greene-Wu, [9], [10]) Let (M, h, i) be a non-compact Riemannian manifold. 1 2m+1 2m+1 Then there exists a proper imbedding f = f , ..., f : M → R whose components A f : M → R are harmonic functions. Remark 3 (minimality) Greene-Wu result is a reﬁned version of the celebrated imbedding theorem by H. Whitney. We point out that, in general, the harmonic imbedding f is not isometric. Namely, f induces on M a metric which may be diﬀerent from the background metric: ∗ f can 2m+1 6= h, i . R N A harmonic, isometric immersion f : M → R is called a minimal immersion. In general a complete manifold M cannot be minimally immersed into any Euclidean space.

Remark 4 (exhaustion) As a consequence of the Theorem we have that there is no obstruc- tion for a generic manifold to support a (strictly) subharmonic exhaustion function. Simply take τ = P f A2 .

Remark 5 (non-linear curiosity) A non-linear generalization of the Laplace operator is rep- resented by the p (> 1)-laplacian

p−2 ∆pu = div |∇u| ∇u .

It is not known whether a generic complete manifold (M, h, i) supports any p-harmonic function. The most general result (avoiding curvature conditions) requires that M is disconnected at inﬁnity (see below for the deﬁnition).

Thus, every non-compact (M, h, i) supports a lot of harmonic functions, independently of its geometry. The geometry of M (volume growth, curvature bounds, topology etc...) enters the game when we investigate harmonic functions with special properties such as Lp-integrability, given sign (semi-boundedness), finite Lp-energy. By way of example, Li-Tam, [15], discovered that positive and finite L2-energy harmonic functions are intimately related to the topology at infinity of the underlying complete manifold (M, h, i), i.e., to the number of its ends. Let Ω be a compact domain of M. An end of M with respect to Ω is any unbounded connected component of M − Ω. Note that the number of ends with respect to Ω is necessarily finite. Clearly, such a number increases with Ω. If it stabilizes to a certain b ∈ N as Ω % M we say that M has b ends. Otherwise, M has infinitely many ends. A manifold with at least two ends is said to be disconnected at infinity. The following result in the setting of minimal submanifolds gives the flavor of Li-Tam harmonic function theory. We shall come back to it later on.

m m+1 Theorem 6 (Cao-Shen-Zhu, [5], Li-Wang, [16]) Let f :(M , h, i) → R be a complete, ∞ ∞ minimal hypersurface. Set HD (M) for the vector space of L -harmonic functions u on M with 2 ∞ ﬁnite Dirichlet integral |∇u| ∈ L . If dim HD (M) < +∞ then M has a ﬁnite number of ends. Moreover ∞ #ends ≤ dim HD (M) .

4 The present talk aims to present a brief survey of some classical and some very recent results about the interplay between existence, non-existence and ﬁnite dimensionality of special harmonic functions and the geometry of the underlying manifold. Obviously, we will not give a com- prehensive view. For instance, we shall not touch important topics such as the L2-cohomological theory related to harmonic functions with ﬁnite L2-energy, splitting-type theorems, Colding- Minicozzi works on Yau conjecture about harmonic functions of polynomial growth and the subsequent developments, harmonic functions on manifolds with special structures, e.g., Kahler manifolds etc...

2 Lp<+∞-harmonic functions

m Let us ﬁrst consider the well known situation of (R , can) . p m Let u be a harmonic function satisfying u ∈ L (R ) for some 1 ≤ p < +∞ . Then, |u (x)| ≥ 0 m is subharmonic and by the standard mean value inequality we have, for any x ∈ R , R |u| |u (x)| ≤ BR(x) . vol (BR (x)) Using H¨older inequality, the latter implies the Lp-mean value inequality R |u (x)|p |u (x)|p ≤ BR(x) . vol (BR (x)) Therefore, taking the limit as R → +∞, we conclude u ≡ 0. Let us now consider the case of a generic complete, non-compact manifold (M, h, i). In general, without any further requirement on the geometry of M, there is no global(!) Lp-mean value inequality.

2.1 Yau vanishing result and Li-Schoen counter-examples In the range 1 < p < +∞ we have the following surprising Lp-Liouville type result. The proof is completely elementary and uses just the divergence theorem (in this sense it can be considered a non-linear proof). The reader should compare Yau’s argument with those supplied in a paper by Greene-Wu, [11] (where, in fact, they required sectional curvature conditions).

Theorem 7 (Yau, [23]) Let (M, h, i) be any complete Riemannian manifold (no assumptions!). Let u be a harmonic function (positive subharmonic function) satisfying u ∈ Lp, some 1 < p < +∞. Then, u is constant. In particular, if vol (M) = +∞, u ≡ 0.

In case p = 1, more generally in the range 0 < p ≤ 1, the above result fails to hold. Counter- examples ﬁrst appeared in a paper by Li-Schoen, [14], and involved negatively curved, complete manifolds with fast sectional curvature decay. Precisely, they constructed 2-dimensional surfaces with Sec −r (x)2+ε , as r (x) → +∞ and supporting non-constant L1-harmonic functions. They also constructed 2-dimensional surfaces with Sec ∼ −Cr (x)−2 , as r (x) → +∞ for large enough C > 0, possessing non-constant Lp-harmonic functions with 0 < p < 1. Here r (x) = dist (x, o) for some ﬁxed origin o ∈ M.

5 2.2 Li-Schoen Lp-mean value inequalities To deal with 0 < p ≤ 1 situations it is quite natural to investigate on the geometric assumptions implying Lp-mean value inequalities.

Theorem 8 (Li-Schoen, [14]) Let (M, h, i) be a complete manifold and o ∈ M a ﬁxed origin. Assume that Ric ≥ − (m − 1) k on BR (o) , for some k ≥ 0. Let v ≥ 0 be a subharmonic function on BR (x) . For any p > 0 there exists a constant C = C (m, p) > 0 such that

√ R vp sup vp ≤ τ −C(1+ kR) BR(x) vol (B (o)) B(1−τ)R(o) R for any τ ∈ (0, 1/2).

Note that, in case Ric ≥ 0, a result Calabi and Yau gives vol (BR (o)) ≥ cR so that we can take limits as R → +∞ and immediately conclude v = 0. In particular, a complete manifold of non-negative Ricci curvature supports no, non-constant L0

Theorem 9 (Li-Schoen, [14]) Let (M, h, i) be a complete manifold. Let r (x) = dist (x, o) for some ﬁxed origin o ∈ M.

1. Assume that (1 + r (x))2 Ric ≥ −C , logα 1 + r (x)2

for r (x) >> 1 and for some constants α, C > 0. Then, every L1-harmonic function on M must be constant.

2. Assume that C Ric ≥ − r (x)2 for r (x) >> 1, and for some C = C (m) > 0 suﬃciently small. Then, every Lp-harmonic function on M, 0 < p < 1, must be constant.

3 (Semi-)Bounded harmonic functions

Say that a function u on M is semibounded if either infM u > −∞ or supM u < +∞. Again, m let us ﬁrst consider the case of R . Suppose that u is a semibounded, harmonic function. For instance, assume infM u > −∞. Take

v = u − inf u. m R

6 Since v is harmonic and satisﬁes inf v = 0 m R the well known Harnack inequality tells us that

0 ≤ sup v ≤ c inf v = 0. m m R R m m It follows that u ≡ infR u, that is, every semibounded harmonic function on R is constant. On a generic complete Riemannian manifold, positive harmonic functions do not enjoy any (global!) Harnack inequality. In a sense, the validity of global Harnack inequalities is a peculiarity of almost-Euclidean spaces. This is true from both the metric (Poincar`e+Doubling) and the curvature point of view.

3.1 Parabolicity and volumes The classical maximum principle asserts that a bounded-above subharmonic function on M which achieves its maximum at some interior point must be constant. It is readily seen that, ∗ equivalently, if a non constant u attains its maximum u at some x0 ∈ M then, for every small ε > 0, inf ∆u < 0. {u>u∗−ε} ∗ Indeed, let Ωε be the connected component of the open set {u > u − ε} containing x0. Thus, ∗ Ωε is an open set. If ∆u ≥ 0 on Ωε, then the maximum principle forces u = u on Ωε. In particular, Ωε is closed. By connectedness Ωε = M and u is a constant function. Now, suppose u is just bounded above and “achieves its supremum at inﬁnity”. What can we say about ∆u? Answering this question leads to the concept of parabolic (and stochastically complete) manifolds. A manifold (M, h, i) is said to be parabolic if every subharmonic function on M bounded from above must be constant:

∆u ≥ 0 =⇒ u = const. supM u < +∞

In particular a parabolic manifold does not support any semibounded harmonic function. The concept of parabolic manifold is related to a wide class of equivalent properties of M involving the Green kernel, the linear capacity, the Brownian motion, etc... Here, we limit ourselves to point out the following maximum principle characterization.

Theorem 10 (M, h, i) is parabolic if and only if for every non constant function u satisfying ∗ u = supM u < +∞ it holds inf ∆u < 0 {u>u∗−ε} for every suﬃciently small ε > 0.

From the geometric point of view, parabolicity of a complete manifold is tightly related to the growth rate of the volume of geodesic balls. The following result is independently due to Ahlfors and Nevanlinna for surfaces, and to Lyons-Sullivan and Grigor’yan for general Riemannian manifolds; see [8].

7 Theorem 11 Let (M, h, i) be complete. If

1 t 7−→ ∈/ L1 (+∞) vol (∂Bt) then M is parabolic.

2 Thus, for instance, the plane R is parabolic. It can be shown that, for rotationally symmetric Riemannian metrics, the above volume condition is also necessary for the manifold to m≥3 be parabolic. Thus, R is not parabolic. Nonlinear proof of Theorem 11 (Rigoli-Setti, [20]). Let u be a subharmonic function ∗ ∗ satisfying u = supM u < +∞. Up to an additive constant we can assume u = 0. Consider the u vector ﬁeld Z = e ∇u. Apply the divergence theorem on a geodesic ball Br and elaborate:

Z Z ∂ Z Z eu |∇u| ≥ eu ∇u, = divZ ≥ eu |∇u|2 . ∂Br ∂Br ∂r Br Br Note that Z Z 1/2 Z 1/2 eu |∇u| ≤ eu |∇u|2 eu . ∂Br ∂Br ∂Br Therefore Z 1/2 Z 1/2 Z eu |∇u|2 eu ≥ eu |∇u|2 . ∂Br ∂Br Br Set Z H (r) = eu |∇u|2 Br and note that, by the co-area formula, Z H0 (r) = eu |∇u|2 . ∂Br The above rewrites H0 (r) 1 ≥ . 2 R eu H (r) ∂Br Integrating on [R, r], and recalling that u ≤ 0, gives

1 1 1 Z r 1 Z r 1 ≥ − ≥ ≥ . H (R) H (R) H (r) R eu vol (∂B ) R ∂Bt R t

Letting r → +∞ we conclude H (R) = 0, that is, |∇u| = 0 and u is constant.

3.2 Structure at inﬁnity Suppose we are given a complete manifold (M, h, i). Denote by H+ (M) the vector space spanned by the positive harmonic functions of M. Also, introduce H0 (M) the vector space spanned by the set of harmonic functions u with the following property: there is a compact domain Ω such that u is semibounded on each of the (ﬁnitely many) ends of M with respect of Ω. Then, we have the following

8 Theorem 12 (Li-Tam, [15]) Assume dim H0 (M) < +∞. Then #ends (M) ≤ dim H0 (M) . Moreover, if M is non-paraobolic and dim H+ (M) < +∞ then #ends (M) ≤ dim H+ (M) . Using this result, Li-Tam obtained ﬁniteness of the number of ends for manifolds with asymptotically-nonnegative Ricci curvature. Theorem 13 (Li-Tam, [15]) Let the complete manifold (M, h, i) satisfy Ric ≥ −k (r (x))

0 for some 0 ≤ k ∈ C (R+) such that tm−1k (t) ∈ L1 (+∞) . Then #ends (M) < +∞.

3.3 Negatively curved manifolds It is readily seen that there are negatively curved manifolds supporting a large amount of non- constant, bounded harmonic functions, despite the fact that they possess only one end! (Li-Tam theory is not efficient in negative curvature). For instance, consider the 2-dimensional hyperbolic space of constant Gaussian curvature −1 given by 2 2 , hyp = B , ρ can 2 , H 1 R 2 with can the canonical metric of R ⊃ B1 and 2 ρ (x) = . 1 − |x|2 Note that the hyperbolic Laplacian is related to the Euclidean one by 1 ∆ 2 = ∆ 2 . H ρ2 (x) R 0 Now, for any ϕ ∈ C (∂B1), solve the Dirichlet problem ∆ 2 u = 0 on B R 1 u = ϕ on ∂B1. 2 Then, u is a bounded harmonic function on H . The situation generalizes to (quasi isometric images of) simply connected, complete Rieman- nian manifolds (M, h, i) satisfying −A2 ≤ Sec ≤ −B2 < 0. Anderson, [1], Sullivan, [21] and, with a more direct and simplified argument (which works even in non-linear settings), Anderson-Schoen, [2], proved that we can compactify M via a geometric boundary S (∞), the sphere at infinity, and solve the asymptotic Dirichlet problem as in case of the hyperbolic disk. Note that, in more recent works, the above curvature conditions have been relaxed. In this respect, we limit ourselves to quote [13].

9 3.4 Non-negatively curved manifolds In this realm one meets one of the most fruitful techniques developed by Yau (and Cheng). Manifolds of non-negative Ricci curvature enjoy global Harnack inequalities. Yau obtained such a conclusion as an integrated form of his famous gradient-estimates for positive harmonic functions. The latter are proved as a clever application of the usual maximum principle for bounded-above functions attaining their maximum. Theorem 14 (Yau, [22]) Let (M, h, i) be a complete manifold satisfying Ric ≥ −k for some constant k ≥ 0. Suppose that u is a positive harmonic function on M. Then, for every geodesic ball BR (x), it holds √ |∇u| 1 + R k ≤ C on B (x) u R R/2 where C > 0 is a constant depending only on m. In particular, √ sup u ≤ e2C(1+R k) inf u. B (x) BR/2(x) R/2 Accordingly, non-negatively Ricci curved manifold do not support any non-constant, semibounded, harmonic function.

4 Harmonic functions with ﬁnite Lp-energy

m m Once again, let us first consider the Euclidean case R . Take a harmonic function u on R . Easy computations show that ∆ |∇u|2 = 2 |Hess (u)|2 . p m 2 Therefore, if |∇u| ∈ L (R ) for some p > 0, by the mean value inequality applied to |∇u| we have |∇u| = 0 and u is a constant function. What happens on a generic manifold? The following result goes back to Wietzenbock in the 20’s and was independently rediscovered and systematically used by Bochner in 50’s. Theorem 15 (Bochner-Weitzenbock formula) Lat u be a harmonic function on a manifold (M, h, i). Then 1 ∆ |∇u|2 = |Hess (u)|2 + Ric (∇u, ∇u) . 2 Furthermore the following refined Kato inequality holds 1 |Hess (u)|2 ≥ 1 + |∇ |∇u||2 . m − 1 Thus, in particular, if Ric ≥ −q (x) for some function 0 ≤ q (x) ∈ C0 (M), one has (in the sense of distributions) 1 |∇u| (∆ |∇u| + q (x) |∇u|) ≥ |∇ |∇u||2 . m − 1 It follows from this result and from what we said in the previous sections that, in case Ric ≥ 0, there are no non-constant harmonic functions with finite Lp-energy, p > 0. This confirms once more the impression that a Ric ≥ 0-manifold really looks like Euclidean space. However, in general, the contribution of the Ricci curvature heavily influences the analysis.

10 4.1 An example in negative curvature Suppose we are given a 2-dimensional manifold (M, h, i). We set L2H (M) for the vector space of L2 harmonic functions on M. Then L2H (M) depends only on the conformal class of h, i. Indeed, if hf, i = λ2 (x) h, i for some smooth function λ (x) > 0, we have

Z Z 2 2 |∇u| dvol = ∇e u dvol,f M M hf,i that is, the Dirichlet integral is a conformal invariant. Furthermore, we have already observed that 1 ∆e = ∆ λ2 (x) so that harmonicity is a conformal invariant. The claim is proved. 2 2 Now, consider the 2-dimensional Hyperbolic space , hyp = B , ρ can 2 of constant H 1 R Gaussian curvature −1. Obviously, since B1 has ﬁnite Euclidean volume, we have

2 dim L H (B1) = +∞.

It follows that 2 2 dim L H H = +∞.

4.2 Spectral assumptions: vanishing and ﬁniteness results The problem is now to ﬁnd a way to take into account the contribution of the (negative part of the) Ricci tensor. Assume Ric ≥ −q (x) for some function q (x) ≥ 0. A possible strategy consists in considering spectral properties of the geometric Schrodinger operator

L = −∆ − q (x)

In the geometric realm, this strategy has its roots in the classical theory of minimal (hy- per)surfaces of the Euclidean space (where L = −∆ − |II|2 is the stability operator). Recall that the bottom of the spectrum of L is deﬁned by

R |∇ϕ|2 − q (x) ϕ2 λL (M) = inf M . 1 ∞ R 2 06=ϕ∈Cc (M) M ϕ

In a 1988 paper, Elworthy-Rosenberg, [6], first observed that on a complete manifold with Ric ≥ −k, condition L λ1 (M) ≥ 0 forces the constancy of harmonic functions (vanishing of harmonic forms) with finite L2-energy. Their technique relies on very refined probabilistic tools. The Ricci curvature assumption is in order to guarantee the completeness of M with respect to the Brownian paths. Soon later, Berard, [3], generalized Elworthy-Rosenberg results by removing the curvature condition. More- over, his proof is completely elementary and makes a direct use of the spectral assumption. His direct approach, however, forces him to consider only L2-energies. What about other energies?

11 It should be noted that one can also be given a manifold (M, h, i) such that the above spectral property is satisﬁed just outside some compact set K ⊂ M, namely,

L λ1 (M − K) ≥ 0. Think for instance of a condition like Ric ≥ 0 on M − K. Classical works by (Gulliver and) Fischer-Colbrie, [7], in minimal surface theory, show that this condition is related (implied) by the ﬁniteness of the (generalized) Morse index Ind (L,M) of L. Recall that

Ind (L,M) = sup Ind (LΩ) Ω⊂⊂M ∂Ω∈C∞ ∞ where LΩ denotes the Friedrichs extension of (L,Cc (Ω)) and Ind (LΩ) is the (finite) number, counting multiplicities, of negative eigenvalues of LΩ. Carron, [4], (in the presence of a Sobolev type inequality) and Li-Wang, [17], (general case) recently obtained that this weaker spectral condition gives finite dimensionality results for the space of harmonic functions (forms) with finite L2-energy. Very recently, using quite different techniques (that allowed us to avoid the use of refined Kato inequalities), we were able to obtain extensions of both vanishing and finiteness result in the Lp-case. In fact, our results apply to more general geometric situations which are governed by a Bochner-type differential inequality. In the following statement, for any q ≥ 1, LqH (M) stands for the vector space of harmonic functions u satisfying |∇u| ∈ Lq.

Theorem 16 (P.-Rigoli-Setti, [18], [19]) Let (M, h, i) be a complete manifold of dimension m > 3 satisfying Ric ≥ −q (x) 0 m−2 for some 0 ≤ q (x) ∈ C (M). Assume that, for some H ≥ m−1 λ−∆−Hq(x) (M) ≥ 0.

m−2 Then, for every m−1 < p ≤ H dim L2pH (M) = 1. In case λ−∆−Hq(x) (M − K) ≥ 0 for some compact set K ⊂ M, then there is a constant N ≥ m depending only on H and on the geometry of M in a neighborhood of K such that

dim L2pH (M) ≤ N < +∞,

m−2 for every m−1 ≤ p ≤ H. Remark 17 In case m ≤ 3, the result still holds assuming that 1/2 < p ≤ H.

By way of example, let us consider the following consequence. By Greene-Wu theorem, every manifold (M, h, i) imbeds into some Euclidean space via a harmonic immersion f. Greene-Wu did not introduce any control on the energy density

2 ∗ |df| = trh,if can of the immersion. Such a request could require higher dimensional Euclidean ambient spaces. . We have the following Bernstein-type result.

12 m−2 1 Theorem 18 Let (M, h, i) be a complete manifold satisfying, for some H ≥ m−1 > 2 and some compact set K ⊂ M, λ−∆−Hq(x) (M − K) ≥ 0, where q (x) denotes the pointwise lower Ricci curvature eigenvalue. Then, there is a constant N ≥ m depending only on H and on the geometry of M in a neighborhood of K such that the T >N following holds. Let f : M → R be a harmonic immersion whose energy density

2 ∗ p |df| = trh,if can ∈ L (M)

m−2 N T for some m−1 ≤ p ≤ H. Then f (M) ⊂ A , an N-dimensional aﬃne subspace of R .

4.3 Structure at infinity Harmonic functions with finite Dirichlet integral can be used to count special ends of the manifold. Suppose we are given an end E, with smooth boundary ∂E, of the complete manifold (M, h, i). Say that the end E is parabolic if the double of E is a parabolic manifold (without boundary). Similarly, one defines non-parabolic ends. A nice result by Cao-Shen-Zhu, [5], gives the following description of non-parabolic ends in the presence of Sobolev-type inequalities.

Theorem 19 (Cao-Shen-Zhu, [5]) Suppose (M, h, i) supports the L2-Sobolev inequality

1 Z µ Z 2µ 2 ∞ u ≤ S2 |∇u| , ∀u ∈ Cc (M) , M M for some µ > 1 and some S2 > 0. Let E be an end of M. Then either E has ﬁnite volume or E is non-parabolic.

In particular, assume that (M, h, i) supports an L1-Sobolev inequality (hence an L2 one)

1 Z µ Z µ ∞ |u| ≤ S1 |∇u| , ∀u ∈ Cc (M) , M M for some µ > 1 and S1 > 0. A classical result by Federer-Fleming states that the latter is equivalent to the isoperimetric inequality

1 {volΩ} µ ≤ S1vol∂Ω which in turn, once integrated, implies

1 +1 volBr (x) ≥ Cr µ , with C > 0 an absolute(!) constant. It follows that every end of M has infinite volume and, therefore, is non-parabolic. By way of example, we know from Hoffman-Spruck, [12], that a minimal submanifold of a Cartan-Hadamard space enjoys an L1-Sobolev inequality. In fact, according to Carron, see [4] and references therein, the same is true if we replace minimality with the assumption that the mean curvature H of the submanifold satisfies |H| ∈ Lm (M). Therefore, in this Setting, only non-parabolic ends appear. The theory of Li-Tam alluded to in the previous sections also relates non-parabolic ends to harmonic functions with finite Dirichlet integral.

13 ∞ Theorem 20 (Li-Tam, [15]) Let (M, h, i) be a complete manifold. Set HD (M) for the vector space of L∞-harmonic functions u on M with ﬁnite Dirichlet integral |∇u| ∈ L2. If ∞ dim HD (M) < +∞ then M has a ﬁnite number of non-paraobolic ends. Moreover ∞ #non-parabolic ends (M) ≤ dim HD (M) .

∞ 2 Obviously HD (M) ⊆ L H (M) and, therefore, we can conclude Proposition 21 Let (M, h, i) be a complete manifold of dimension m ≥ 2 satisfying

Ric ≥ −q (x) for some 0 ≤ q (x) ∈ C0 (M). Assume that

λ−∆−q(x) (M) ≥ 0.

Then, M has only one non-parabolic end. Furthermore, in case

λ−∆−q(x) (M − K) ≥ 0 for some compact set K ⊂ M, then M has ﬁnitely many non-parabolic ends.

References

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