# Talk:Singular measure

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## mutually singular

so why not call this article "mutually singular measures" like the first line says? --itaj 04:01, 12 May 2006 (UTC)

## a question

let ${\displaystyle (\Omega ,\Sigma ),}$ be a measurable space. i'm talking here about real-value signed finite measures on this space. let M be a set of measures.

let N be the set of all measures absolutely continuous with respect to a measure in the linear span of measures in M. i.e. ${\displaystyle N:=\{\nu :\exists \mu \in span(M)\ (\nu <<\mu )\}}$

for a measure ${\displaystyle \mu }$ i'll denote ${\displaystyle {singl}(\mu ):=\{\nu :\nu \perp \mu \}}$. the set of measures mutually singular to ${\displaystyle \mu }$.

and for a set of measures L. ${\displaystyle {Singl}(L):=\{\nu :\forall \mu \in L\ (\mu \perp \nu )\}}$. the set of all measures mutually singular to all measures in L.

my question is if the above implies that ${\displaystyle N=Singl(Singl(N))}$ i know this is true if there's only one measure in M, but i need to know about infinite set M, countable and bigger. --itaj 04:05, 12 May 2006 (UTC)

If span is in the sense of vector spaces, then no. Consider ${\displaystyle M:=\{\delta _{n}:n\in N\}}$. Then ${\displaystyle \sum 2^{-n}\delta _{n}\not \in span(M)}$ or N, but it is in singl(singl(N)). (Cj67 23:39, 25 June 2006 (UTC))