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In Riemannian geometry, the **Cheeger isoperimetric constant** of a compact Riemannian manifold *M* is a positive real number *h*(*M*) defined in terms of the minimal area of a hypersurface that divides *M* into two disjoint pieces. In 1971, Jeff Cheeger proved an inequality that related the first nontrivial eigenvalue of the Laplace–Beltrami operator on *M* to *h*(*M*). In 1982, Peter Buser proved a reverse version of this inequality, and the two inequalities put together are sometimes called the *Cheeger-Buser inequality*. These inequalities were highly influential not only in Riemannian geometry and global analysis, but also in the theory of Markov chains and in graph theory, where they have inspired the analogous Cheeger constant of a graph and the notion of conductance.

Let *M* be an *n*-dimensional closed Riemannian manifold. Let *V*(*A*) denote the volume of an *n*-dimensional submanifold *A* and *S*(*E*) denote the *n*−1-dimensional volume of a submanifold *E* (commonly called "area" in this context). The **Cheeger isoperimetric constant** of *M* is defined to be

where the infimum is taken over all smooth *n*−1-dimensional submanifolds *E* of *M* which divide it into two disjoint submanifolds *A* and *B*. The isoperimetric constant may be defined more generally for noncompact Riemannian manifolds of finite volume.

Jeff Cheeger proved^{[1]} a lower bound for the smallest positive eigenvalue of the Laplacian on *M* in term of what is now called the Cheeger isoperimetric constant *h*(*M*):

This inequality is optimal in the following sense: for any *h* > 0, natural number *k*, and *ε* > 0, there exists a two-dimensional Riemannian manifold *M* with the isoperimetric constant *h*(*M*) = *h* and such that the *k*th eigenvalue of the Laplacian is within *ε* from the Cheeger bound.^{[2]}

Peter Buser proved^{[3]} an upper bound for the smallest positive eigenvalue of the Laplacian on *M* in terms of the Cheeger isoperimetric constant *h*(*M*). Let *M* be an *n*-dimensional closed Riemannian manifold whose Ricci curvature is bounded below by −(*n*−1)*a*^{2}, where *a* ≥ 0. Then

- Buser, Peter (1978). "Über eine Ungleichung von Cheeger".
*Mathematische Zeitschrift*(in German).**158**(3): 245–252. doi:10.1007/BF01214795. ISSN 0025-5874.