A concept of function which measures the distance to a point at infinity. Let be a Riemannian manifold. The Riemannian metric induces a distance function on . Let be a ray in , i.e., a unit-speed geodesic line such that for all . The Busemann function with respect to is defined by
Since is bounded above by and is monotone non-decreasing in , the limit always exits. It follows that is a Lipschitz function with Lipschitz constant . The level surfaces of a Busemann function are called horospheres. Busemann functions can also be defined on intrinsic (or length) metric spaces, in the same manner. Actually, H. Busemann [a2] first introduced them on so-called -spaces and used them to state the parallel axiom on straight -spaces (cf. also Closed geodesic).
If has non-negative sectional curvature, is convex, see [a3]. If has non-negative Ricci curvature, is a subharmonic function, see [a4]. If is a Kähler manifold with non-negative holomorphic bisectional curvature, is a plurisubharmonic function, see [a7]. If is a Hadamard manifold, is a concave function, see [a9], [a2], and, moreover, the horospheres are -hypersurfaces, see [a9]. On the Poincaré model of the hyperbolic space, the horospheres coincide with the Euclidean spheres in which are tangent to the sphere at infinity. On Hadamard manifolds, it is more customary to call the Busemann function instead of .
More recently, M. Gromov [a1] introduced a generalization of the concept of Busemann function called the horofunction. Let be the set of continuous functions on and let the quotient space of modulo the constant functions. Use the topology on induced from the uniform convergence on compact sets and its quotient topology on . The embedding of into defined by induces an embedding . The closure of the image is a compactification of (cf. also Compactification). According to [a1], [a8], a horofunction is defined to be a class (or an element of a class) in the topological boundary of in . Any Busemann function is a horofunction. For Hadamard manifolds, any horofunction can be represented as some Busemann function, see [a1]. However, this is not necessarily true for non-Hadamard manifolds. Horofunctions have been defined not only for Riemannian manifolds but also for complete locally compact metric spaces.
Let be a complete non-compact Riemannian manifold with non-negative sectional curvature, and for , let , , where runs over all rays emanating from . Then, is a convex exhaustion function, see [a3], that is, a function on such that is compact for any . The function plays an important role in the first step of the Cheeger–Gromoll structure theory for , see [a3]. Any Kähler manifold admitting a strictly plurisubharmonic exhaustion function is a Stein manifold, see [a5]. This, together with the use of Busemann functions or , yields various sufficient conditions for a Kähler manifold to be Stein; see, for example, [a7], [a17]. Some results for the exhaustion property of Busemann functions are known, see [a14], [a15], [a12], [a13]. For a generalization of the notion of a horofunction and of , see [a18]. A general reference for Busemann function and its related topics is [a16].
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Busemann function. T. Shioya (originator), Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Busemann_function&oldid=13719