zbMATH — the first resource for mathematics

Geometry Search for the term Geometry in any field. Queries are case-independent.
Funct* Wildcard queries are specified by * (e.g. functions, functorial, etc.). Otherwise the search is exact.
"Topological group" Phrases (multi-words) should be set in "straight quotation marks".
au: Bourbaki & ti: Algebra Search for author and title. The and-operator & is default and can be omitted.
Chebyshev | Tschebyscheff The or-operator | allows to search for Chebyshev or Tschebyscheff.
"Quasi* map*" py: 1989 The resulting documents have publication year 1989.
so: Eur* J* Mat* Soc* cc: 14 Search for publications in a particular source with a Mathematics Subject Classification code (cc) in 14.
"Partial diff* eq*" ! elliptic The not-operator ! eliminates all results containing the word elliptic.
dt: b & au: Hilbert The document type is set to books; alternatively: j for journal articles, a for book articles.
py: 2000-2015 cc: (94A | 11T) Number ranges are accepted. Terms can be grouped within (parentheses).
la: chinese Find documents in a given language. ISO 639-1 language codes can also be used.

a & b logic and
a | b logic or
!ab logic not
abc* right wildcard
"ab c" phrase
(ab c) parentheses
any anywhere an internal document identifier
au author, editor ai internal author identifier
ti title la language
so source ab review, abstract
py publication year rv reviewer
cc MSC code ut uncontrolled term
dt document type (j: journal article; b: book; a: book article)
Comparing the Floyd and ideal boundaries of a metric space. (English) Zbl 1182.54030

Let (X,d) be a metric space. In X we can define different types of boundaries: the Ideal boundary of X denoted by 1 X; the Gromov boundary, denoted by G X; the g-boundary, denoted by g X.

The Ideal boundary is defined using geodesic rays. The Gromov boundary is defined through Gromov sequences (sequences converging to infinity). The Gromov Boundary is usually defined only for Gromov hyperbolic spaces but the authors extended this concept to arbitrary metric spaces. The g-boundary is the boundary of an unbounded length space which replaces an unbounded metric l by a bounded metric σ, using a conformal distortion involving a function g.

Actually spherical and Floyd boundaries are types of g-boundaries. In fact, let g:[0,)(0,) be a measurable function such that for a constant C>2 one can establish the following conditions (where G(t):= 0 g(s)ds,t0):

(F1) g(t)Cg(s) when s,t0 and s-1t2s+1,

(S1) g(t)Cg(s) when s,t0 and (s-1)/2t2s+1,

(F2) G(0)Cg(0),

(S2) G(t)C(1+t)g(t), t0,

g is a weak C-Floyd function if conditions (F1) and (F2) are satisfied, g is a Floyd function if (S1) and (F2) are satisfied and g is a C-sphericalizing function if (S1) and (S2) are satisfied. The spherical boundary was a consequence of the concept of sphericalization introduced in by Z. Balogh and S. M. Buckley [Conform. Geom. Dyn. 9, 76–101, electronic only (2005; Zbl 1108.30033)]. The purpose was to interpret quasyhyperbolizations of unbounded length spaces in the context of unbounded space. In the article under review the authors aim to discuss g-boundaries comparing them with ideal and Gromov boundaries in order to emphasize both similarities and differences. Some of the results shown are also found in the literature available but the authors prove comparison results for larger classes of spaces X and functions g. The article is organized in three sections. The first, notation and preliminaries, includes a section on CAT(0) and Gromov hyperbolic spaces and one on spherical and Floyd boundaries, for, as the authors say in the introduction, understanding the spherical boundary is vital for deciding when sphericalization is an invertible process, (for more details see the article mentioned above). The second section explains the main comparison theorems. It includes five comparison theorems with proof.

The authors include also one last section with an extensive list of counterexamples to show that the assumptions given in the comparison theorems are indeed needed.

54D35Extensions of topological spaces (compactifications, supercompactifications, completions, etc.)
30F45Conformal metrics (hyperbolic, Poincaré, distance functions)
53A30Conformal differential geometry