## Rocky Mountain Journal of Mathematics

### Characterizations of the compactness of Riemannian manifolds by eigenfunctions, and a partial proof of a conjecture by Hamilton

Xiang Gao

#### Abstract

In this paper, we deal with comparison theorems for the first eigenvalue of the Schr\"{o}dinger operator, and we present some criteria for the compactness of a Riemannian manifold in terms of the eigenfunctions of its Laplacian. Firstly, we establish a comparison theorem for the first Dirichlet eigenvalue $\mu _1^D (B (p,r))$ of a given Schr\"{o}dinger operator. We then prove that, for the space form $M_K^n$ with constant sectional curvature $K$, the first eigenvalue of the Laplacian operator $\lambda _1 (M_K^n)$ is greater than the limit of the corresponding first Dirichlet eigenvalue $\lambda _1^D(B_K(p,r))$. Based on these, we present a characterization of a compact gradient shrinking Ricci soliton locally being an $n$-dim space form by the first eigenfunctions of the Laplacian operator, which gives a generalization of an interesting result by Cheng \cite {4} from 2-dim to $n$-dim. This result also gives a partial proof of a conjecture by Hamilton \cite {7} that a compact gradient shrinking Ricci soliton with positive curvature operator must be Einstein. Finally, we derive a criterion of the compactness of manifolds, which gives a partial proof of another conjecture by Hamilton \cite {6} that, if a complete Riemannian 3-manifold $(M^3, g)$ satisfies the Ricci pinching condition $Rc \ge \varepsilon Rg$, where $R > 0$ and $\varepsilon$ is a positive constant, then it is compact. In fact, our result is also true for general $n$-dim manifolds.

#### Article information

Source
Rocky Mountain J. Math., Volume 46, Number 2 (2016), 461-480.

Dates
First available in Project Euclid: 26 July 2016

https://projecteuclid.org/euclid.rmjm/1469537473

Digital Object Identifier
doi:10.1216/RMJ-2016-46-2-461

Mathematical Reviews number (MathSciNet)
MR3529079

Zentralblatt MATH identifier
1345.35065

Subjects
Primary: 35P05: General topics in linear spectral theory 58G25

#### Citation

Gao, Xiang. Characterizations of the compactness of Riemannian manifolds by eigenfunctions, and a partial proof of a conjecture by Hamilton. Rocky Mountain J. Math. 46 (2016), no. 2, 461--480. doi:10.1216/RMJ-2016-46-2-461. https://projecteuclid.org/euclid.rmjm/1469537473

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