Let $B$ be a Boolean Algebra. 


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.


Is there a strictly positive measure on every countable Boolean Algebra?



Strictly Positive Measures on Countable Boolean Algebras
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.




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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.


