Strictly Positive Measures on Countable Boolean Algebras - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T01:31:09Z http://mathoverflow.net/feeds/question/99808 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/99808/strictly-positive-measures-on-countable-boolean-algebras Strictly Positive Measures on Countable Boolean Algebras provocateur 2012-06-16T21:31:10Z 2012-06-16T22:11:24Z <p>Let $B$ be a Boolean Algebra. </p> <p>A strictly positive measure on $B$ is a function $m$ from $B$ to $[0,1]$ such that (i) $m(b)=0$ iff $b=0$, (ii) $m(1)=1$, and (iii) $m(a+b)=m(a)+m(b)$ whenever $a$ and $b$ are disjoint.</p> <p>Is there a strictly positive measure on every countable Boolean Algebra?</p> http://mathoverflow.net/questions/99808/strictly-positive-measures-on-countable-boolean-algebras/99809#99809 Answer by François G. Dorais for Strictly Positive Measures on Countable Boolean Algebras François G. Dorais 2012-06-16T22:00:35Z 2012-06-16T22:11:24Z <p>Yes. Let $M$ be the space of all measures on $B$. This is a compact space when endowed with with the pointwise convergence topology since it is a closed subspace of $[0,1]^B$. If $b$ is a nonzero element of $B$, then the set $U_b = \lbrace m \in M : m(b) \gt 0 \rbrace$ is open and dense in $M$. By the Baire Category Theorem, the intersection of all these sets is nonempty.</p> <hr> <p>For an explicit construction, let $b_1,b_2,\ldots$ enumerate $B\setminus\lbrace0\rbrace$ and for each $n$ let $m_n$ be a measure on $B$ such that $m_n(b_n) \gt 0$ (e.g. let $m_n$ be the characteristic function of an ultrafilter containing $b_n$). Then $m = \sum_{n=1}^\infty 2^{-n}m_n$ is as required.</p>