- Research Article
- Open Access

- En-Bing Lin
^{1}Email author

**2010**:346308

https://doi.org/10.1155/2010/346308

© En-Bing Lin 2010

**Received:**31 December 2009**Accepted:**7 February 2010**Published:**4 March 2010

## Abstract

We prove a characterization of a nonhomogeneous *A*-harmonic equation and describe its generalization. We also point out its connection with 1-Harmonic equation.

## Keywords

- Continuous Function
- Weak Solution
- Open Problem
- Usual Sense
- Harmonic Extension

## 1. Introduction

Both *A*-harmonic equations and
-harmonic geometry are rich subjects [1–5]. Many results on both topics have been derived, respectively, but there are very few papers relating both subjects. In this paper, we will connect these two subjects by extending several results from 1-Harmonic functions to *A*-Harmonic functions.

We consider the following setting: a
function
is said to be *A*-harmonic if it is a weak solution of *A*-harmonic equation

where is the length of the gradient of , and for a function without a critical point, is said to be the 1-tension field of .

Let be an open subset of . Consider the following second-order divergence-type elliptic equation:

where are two fixed constants, and is called a weight if and a.e. Also, in general where is a weight.

In this paper, we characterize subsolutions of (1.1) and indicate its generalization to (1.2). We first recall some results in homogeneous *A*-harmonic equations in the following section, followed by the main results in Section 3. Some open problems are discussed in the last section.

## 2. Homogeneous -Harmonic Equations

Definition.

It was shown in [4] that very weak solutions of (1.2) in fact weak solutions of (1.2) in the usual sense.

Definition 2.2.

an -harmonic tensor in of satisfies the -harmonic equation (2.1) in .

Consider the space of differential
-forms
being an *A*-harmonic tensor in a domain
, and
Assume that
, and
for some
. Then the following local weighted Poincare inequality for *A*-harmonic tensors was proved in [3]. There exists a constant
independent of
such that

for all balls with , where the measure is defined by , and is the ball with the same center as and with .

## 3. Characterizations of Nonhomogeneous -Harmonic Equations

In what follows, we prove an *A*-harmonic analog of 1-harmonic equations.

Lemma 3.1.

Let M be a complete noncompact Riemannian. For any and any pair of positive numbers with , there exist a rotational symmetric Lipschitz continuous function and a constant (independent of ) with the following properties:

(i) on B( ; s) and off B ( ; s),

Proof.

See Andreotti and Vesentini [6], Yau [7], and Karp [8].

Theorem 3.2.

then the infimum c satisfies , where k only depends on , and .

Proof.

Corollary 3.3.

Let be a weak subsolution of -harmonic equation (1.1) with constant 1-tension field , that is, in the distribution sense. Then is an A-harmonic function.

Remark.

In a similar fashion, the above results can be extended to the nonhomogeneous equation by using Sobolev Imbedding Theorem.

## 4. Further Discussions

It would be interesting to find similar results of Section 2 for nonhomogeneous -harmonic equations. It would also be interesting to seek analogs of 1-harmonic applications in calibration geometry. The extension of 1-harmonic functions to -harmonic functions on hyperbolic spaces and their associated spaces could be explored.

To conclude this paper, we state another -harmonic extension of 1-harmonic result [5], that is an immediate consequence of Corollary 3.3.

Theorem.

Let , and for every x in . The following statements are equivalent.

(i) is a weak subsolution of (1.1) with constant 1-tension field.

## Declarations

### Acknowledgments

The author wishes to express sincere gratitude to Professor S. Walter Wei for many helpful suggestions and encouragements without which this would have not been written.

## Authors’ Affiliations

## References

- Agarwal RP, Ding S: Advances in differential forms and the
*A*-harmonic equation.*Mathematical and Computer Modelling*2003, 37(12–13):1393–1426. 10.1016/S0895-7177(03)90049-5MathSciNetView ArticleMATHGoogle Scholar - Ding S: Integral estimates for the Laplace-Beltrami and Green's operators applied to differential forms on manifolds.
*Zeitschrift für Analysis und ihre Anwendungen*2003, 22(4):939–957.View ArticleMATHGoogle Scholar - Ding S, Liu B: Generalized Poincaré inequalities for solutions to the
*A*-harmonic equation in certain domains.*Journal of Mathematical Analysis and Applications*2000, 252(2):538–548. 10.1006/jmaa.2000.6951MathSciNetView ArticleGoogle Scholar - Gao H-Y, Zhang Y, Chu Y-M: Regularity for very weak solutions of -harmonic equation with weight.
*Kyungpook Mathematical Journal*2009, 49(2):195–202.MathSciNetView ArticleMATHGoogle Scholar - Wei SW: On 1-harmonic functions.
*Symmetry, Integrability and Geometry: Methods and Applications*2007, 3: 127–136.MathSciNetMATHGoogle Scholar - Andreotti A, Vesentini E: Carleman estimates for the Laplace-Beltrami equation on complex manifolds. Institut des Hautes Études Scientifiques. Publications Mathématiques 1965, (25):81–130.Google Scholar
- Yau ST: Some function-theoretic properties of complete Riemannian manifold and their applications to geometry.
*Indiana University Mathematics Journal*1976, 25(7):659–670. 10.1512/iumj.1976.25.25051MathSciNetView ArticleMATHGoogle Scholar - Karp L: Subharmonic functions on real and complex manifolds.
*Mathematische Zeitschrift*1982, 179(4):535–554. 10.1007/BF01215065MathSciNetView ArticleMATHGoogle Scholar

## Copyright

This article is published under license to BioMed Central Ltd. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.