Pre-measure: Difference between revisions
m robot Removing: pl:Premiara |
|||
Line 17: | Line 17: | ||
(Note that there is some variation in the terminology used in the literature. For example, Rogers (1998) uses "measure" where this article uses the term "outer measure". Outer measures are not, in general, measures, since they may fail to be [[sigma additivity|''σ''-additive]].) |
(Note that there is some variation in the terminology used in the literature. For example, Rogers (1998) uses "measure" where this article uses the term "outer measure". Outer measures are not, in general, measures, since they may fail to be [[sigma additivity|''σ''-additive]].) |
||
==See also== |
|||
* [[Hahn-Kolmogorov theorem]] |
|||
==References== |
==References== |
Revision as of 21:58, 3 July 2011
In mathematics, a pre-measure is a function that is, in some sense, a precursor to a bona fide measure on a given space. Pre-measures are particularly useful in fractal geometry and dimension theory, where they can be used to define measures such as Hausdorff measure and packing measure on (subsets of) metric spaces.
Definition
Let be a ring of sets and a set function. is called a pre-measure if
and for a sequence of pairwise disjoint sets whose union lies in
- .
The last property is called -additivity
Extension theorem
It turns out that pre-measures can be extended quite naturally to outer measures, which are defined for all subsets of the space . More precisely, if is a pre-measure defined on a ring of subsets of the space , then the set function defined by
is an outer measure on .
(Note that there is some variation in the terminology used in the literature. For example, Rogers (1998) uses "measure" where this article uses the term "outer measure". Outer measures are not, in general, measures, since they may fail to be σ-additive.)
See also
References
- Munroe, M. E. (1953). Introduction to measure and integration. Cambridge, Mass.: Addison-Wesley Publishing Company Inc. p. 310. MR0053186
- Rogers, C. A. (1998). Hausdorff measures. Cambridge Mathematical Library (Third ed.). Cambridge: Cambridge University Press. p. 195. ISBN 0-521-62491-6. MR1692618 (See section 1.2.)
- Folland, G. B. (1999). Real Analysis. Pure and Applied Mathematics (Second ed.). New York: John Wiley & Sons, Inc. pp. 30–31. ISBN 0-471-31716-0.