TITLE: A curiosity on complete homomorphisms of boolean algebras
QUESTION [13 upvotes]: The question may be trivial, but has eluded me, may be it is more appropriate for mathstack-exchange.
Let $B$, $C$ be boolean algebras and $i:B\to C$ be an homomorphism.
By Stone duality to each such $i$ corresponds a continuos map $\pi:St(C)\to St(B)$ defined by $G\mapsto i^{-1}[G]$ for any $G$ ultrafilter on $B$
(where $St(B)$ is the compact $0$-dimensional space of ultrafilters on $B$ with topology given by the base of clopen sets
$N_c$ given by ultrafilters to which $c$ belongs, as $c$ ranges in $B$).
It is easy to check that if $\pi$ is open then $i$ is a complete homomorphism (i.e. it preserves arbitrary suprema whenever they exist in $B$).
Does the converse always hold? 
The converse holds in case $\pi$ induces an order preserving
map $\pi^*:C\to B$ defined by $N_{\pi^*(c)}=\pi[N_c]$ for all $c\in C$,
which occurs for example if $B$ is a complete boolean algebra. 
What if $B$ is not complete and $i$ is complete? In this case I'm stuck.
 Here is a sketch
of the easy implication and of the partial converse:
(1) Assume $i$ is not complete. Let $D$ be a predense subset of $B$
such that $i[D]$ is not predense in $C$. 
Fix $c\in C$ such that $c\wedge i(d)=0$ for all $d\in D$.
Then
$A=\bigcup_{d\in D} N_d$
is a dense open subset of $St(B)$ and $\pi[N_c]\cap A=\emptyset$.
Since $St(B)\setminus A$ is  closed nowhere dense,
this gives that $\pi[N_c]$ is closed (since $St(B)$ is compact Hausdorff) and nowhere dense, 
hence it is not open. 
(2) It suffices to show that $\pi[N_c]$ is clopen for all $c\in C$.
Let $A=\{b\in B: i(b)\wedge c=0\}$. By completeness of $B$, $\bigvee A=d$ exists, and by completeness of $i$, $i(d)\wedge c=0$. 
It is not hard to check that $\pi[N_c]=N_{\neg d}$ (since $G\not\in \pi[N_c]$ if and only if $G\cap A$ is non-empty).

REPLY [5 votes]: By Corollary 6.3 of [1], the morphism $i$ of Boolean algebras is complete if and only if the continuous function $\pi$ is quasi-open, which means that $int(\pi(U))$ is nonempty for every nonempty open subset $U$ of the domain. The example given by Don Monk shows that this is a strictly weaker property than being an open continous map, as one would expect.
[1] Bezhanishvili, Guram, Stone duality and Gleason covers through de Vries duality, Topology Appl. 157, No. 6, 1064-1080 (2010). ZBL1190.54015.
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