Sigma-additive set function: Difference between revisions

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A [[set function]] <math>\mu</math> on a [[family of sets]] <math>\mathcal{S}</math> is called a '''{{visible anchor|modular set function}}''' and a '''[[Valuation (geometry)|{{visible anchor|valuation}}]]''' if whenever <math>A,</math> <math>B,</math> <math>A\cup B,</math> and <math>A\cap B</math> are elements of <math>\mathcal{S},</math> then
<math display="block"> \phimu(A\cup B)+ \phimu(A\cap B) = \phimu(A) + \phimu(B)</math>
The above property is called '''{{visible anchor|modularity}}''' and the argument below proves that additivity implies modularity.
 
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See [[Measure (mathematics)|measure]] and [[signed measure]] for more examples of {{sigma}}-additive functions.
 
A ''charge'' is defined to be a finitely additive set function that maps <math>\varnothing</math> to <math>0.</math><ref>{{Cite book|lastlast1=Bhaskara Rao|firstfirst1=K. P. S.|first2=M. |last2=Bhaskara Rao|url=https://www.worldcat.org/oclc/21196971|title=Theory of charges: a study of finitely additive measures|date=1983|publisher=Academic Press|isbn=0-12-095780-9|___location=London|pages=35|oclc=21196971}}</ref> (Cf. [[ba space]] for information about ''bounded'' charges, where we say a charge is ''bounded'' to mean its range is a bounded subset of ''R''.)
 
===An additive function which is not &sigma;-additive===