Outer Measure
A measure that approximately involves set
from the outside.Definition of outer measure
The set function
defined for every subset 
of space 
satisfies the following conditions:
, 
Definition of Lebesgue measure
If
in the example of additive measure,
Definition of
-measurableIf the outer measure
is defined in space 
, and the following conditions are satisfied with respect to set 
, then 
is 
-measurable.For
, 
-measurable setProperties of
In particular,
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