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Consider the algebra $B=P(\omega)/_{\mathrm{fin}}$ (the quotient of the power set of natural numbers modulo the ideal of finite sets). Is there an infinite strictly descending chain $\{A_i\mid i\in I\}$ of subalgebras of $B$, such that there is an embedding of $A_{i+1}$ into $A_i$, but there is no embedding of $A_i$ into $A_{i+1}$.

2nd EDIT: Klaas Pieter Hart expanded his answer into a paper “Many subalgebras of $P(\omega)/_{\mathrm{fin}}$” that can be found at arXiv.

1st EDIT: Corrected question according to the remark in the comments.

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    $\begingroup$ Your definition of "proper subalgebra" is incorrect. $B\subset C$ proper subalgebra means that $C-B$ is nonempty. Either you're asking the question as asked and then the answer is a trivial yes. Or you're asking that there is no Boolean algebra embedding of $A_i$ into $A_{i+1}$ but then the question should be rephrased. $\endgroup$
    – YCor
    Commented Mar 5, 2023 at 12:29
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    $\begingroup$ Would it be possible for you to provide an instance of such a descending chain of subalgebras perhaps in another Boolean algebra? $\endgroup$ Commented Mar 6, 2023 at 0:25
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    $\begingroup$ @JoelDavidHamkins I cannot come up with any example of Boolean algebra with this property, so maybe I should have started with the more elementary question. The closest I got is an example of Heyting algebras. Start with [0,1] interval treated as a Heyting algebra, and follow the Cantor set construction stages, but delete only the middle third sets of irrationals, instead of the whole intervals. Every $H_{i+1}$ is embeddable in $H_i$, but is not isomorphic. $\endgroup$ Commented Mar 6, 2023 at 8:31
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    $\begingroup$ I like this question. I think it's likely to be tricky because often one shows a non-embedding of $A$ in $C$ by showing some cardinal invariant of the algebra to be smaller in $C$. But of course, we can't decrease a cardinal invariant infinitely often to solve this question. $\endgroup$
    – Will Brian
    Commented Mar 6, 2023 at 10:20
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    $\begingroup$ Since it hasn't been said explicitly: it's easy to check there is no such chain consisting of countable subalgebras (of any Boolean algebra). In ZFC+CH (the most interesting case to my taste, although not for many set theorists, I expect) the question is equivalent to finding such a chain consisting of BAs of continuum cardinal. $\endgroup$
    – YCor
    Commented Mar 6, 2023 at 11:55

1 Answer 1

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There is a family $\{K_X:X\subseteq\mathfrak{c}\}$ of separable compact zero-dimensional spaces such that there is a continuous surjection of $K_X$ onto $K_Y$ if and only if $X\subseteq Y$.

These spaces are continuous images of $\omega^*$, the Stone space of $\mathcal{P}(\omega)/\mathit{fin}$. So Stone duality then yields a family $\{B_X:X\subseteq\mathfrak{c}\}$ of subalgebras of $\mathcal{P}(\omega)/\mathit{fin}$ such that $B_Y$ embeds into $B_X$ if and only if $X\subseteq Y$. Thus one can even get chains of length $\mathfrak{c}$ or a chain of the same order type as the real line.

We can make such spaces using variations on Alexandroff's double-arrow space, which is the set $D=([0,1]\times\{0,1\})\setminus\{\langle0,0\rangle,\langle0,1\rangle\}$ ordered lexicographically and endowed with the order topology; The resulting space will be denoted $\mathbb{A}$. (We drop the points $\langle0,0\rangle$ and $\langle0,1\rangle$ because they would be (the only) isolated points of $\mathbb{A}$.)

Pictorially we have taken the unit interval $[0,1]$ and split all points $x$ of the open interval $(0,1)$ into two copies: The space $\mathbb{A}$ is compact and separable, hence a continuous image of $\omega^*$.

The variations will be obtained by specifying a subset $X$ of $(0,1)$ and taking $\mathbb{A}_X=\{\langle x,i\rangle\in D: x\in X \to i=0\}$; that is, by splitting the points of $(0,1)\setminus X$ only. Thus we can write $\mathbb{A}=\mathbb{A}_\emptyset$, and $[0,1]=\mathbb{A}_{(0,1)}$.

One direction in the if and only if will be immediate by the following observation: if $X\subseteq Y$ then there is a natural continuous map $h_{X,Y}$ from $\mathbb{A}_X$ onto $\mathbb{A}_Y$; it sends the points $\langle x,i\rangle$ to $\langle x,i\rangle$ if $x\notin Y$; the points $\langle x,i\rangle$ to $\langle x,0\rangle$ if $x\in Y\setminus X$; and $\langle x,0\rangle$ to $\langle x,0\rangle$ if $x\in X$.

We shall construct a family $\{A_\alpha:\alpha\in\mathfrak{c}\}$ of subsets of $(0,1)$ (all disjoint from $\mathbb{Q}$) and put $S_X=\mathbb{Q}\cup\bigcup_{\alpha\in X}A_\alpha$ for $X\subseteq\mathfrak{c}$. Then $K_X$ will be $\mathbb{A}_{S_X}$. It is readily seen that $h_{S_X,S_Z}=h_{S_Y,S_Z}\circ h_{S_X,S_Y}$ whenever $X\subseteq Y\subseteq Z$, so the maps dual to the surjections $h_{S_\emptyset,S_X}$ yields an embedding of $\mathcal{P}(\mathfrak{c})$ into the family of subalgebras of the clopen algebra of $K_\emptyset$ ordered by the subset relation and so also into the family of subalgebras of $\mathcal{P}(\omega)/\mathit{fin}$.

To this end we let $\mathcal{F}$ be the set of all maps $f$ that satisfy: $\operatorname{dom}f$ is a co-countable subset of $[0,1]$ and $f:\operatorname{dom}f\to[0,1]$ is continuous. For every $f\in\mathcal{F}$ we let $S(f)=\{x\in\operatorname{dom}f:f(x)\neq x\}$ and $E(f)=\operatorname{dom}f\setminus S(f)$. We choose a subset $C(f)$ of $\operatorname{dom}f$ such that the restriction $f:C(f)\to f[S(f)]$ is a bijection.

We apply Theorem 2.0 of A method for constructing ordered continua, Topology and its Applications, 21 (1985), 35--49 to this family and obtain a pairwise disjoint family $\{V\}\cup\{A_\alpha:\alpha\in\mathfrak{c}\}$ of Bernstein sets in $(0,1)$ with the following properties: all are disjoint from $\mathbb{Q}$ and for every $f\in\mathcal{F}$ such that $f[S(f)]$ (and hence $C(f)$) has cardinality $\mathfrak{c}$ the intersections $C(f)\cap A_\alpha$ and $f[C(f)\cap A_\alpha]\cap V$ have cardinality $\mathfrak{c}$, for all $\alpha$.

For later use we note that for a given function $f$ in $\mathcal{F}$ the sets $E(f)$ and $S(f)$ are closed and open in $\operatorname{dom}f$ respectively and hence completely metrizable, and the set $f[Sf)]$ is analytic. Therefore for each of the three sets we know that either it is countable or it contains a copy of the Cantor set. Thus, to show that one is countable it suffices to show that its intersection with some Bernstein set is countable.

The following claim establishes the other direction in our if and only if.

Let $\alpha\in\mathfrak{c}$ and let $X$ and $Y$ be subsets of $(0,1)$ such that $A_\alpha\subseteq X$ and $(A_\alpha\cup V)\cap Y=\emptyset$. We claim that there is no continuous surjection from $\mathbb{A}_X$ onto $\mathbb{A}_Y$.

To why this claim suffices assume that $U$ and $V$ are subsets of $\mathfrak{c}$ and that $U\not\subseteq V$. Take $\alpha\in U\setminus V$ and note that $A_\alpha\subseteq S_U$ and that $(A_\alpha\cup V)\cap S_V=\emptyset$, so that there is no continuous surjection from $\mathbb{A}_{S_U}$ onto $\mathbb{A}_{S_V}$.

For brevity we write $A=A_\alpha$. We first observe that the points of $A$ are not split in $\mathbb{A}_X$, so its subspace topology is the same as its subspace topology in $[0,1]$.

Now let $s:A_X\to A_Y$ be continuous and let $t:A_Y\to[0,1]$ be the natural map that identifies $\langle y,0\rangle$ and $\langle y,1\rangle$ for all $y$ outside $Y$.

The composition $t\circ s$ is continuous and we let $g$ denote its restriction to $A$. Since $A$ is dense in $\mathbb{A}_X$ we can recover $t\circ s$ from $g$. If $x\in X$ then $(t\circ s)(x,0)=\lim_{y\to x}g(y)$, and if $x\notin X$ then $(t\circ s)(x,0)=\lim_{y\uparrow x}g(y)$ and $(t\circ s)(x,0)=\lim_{y\downarrow x}g(y)$, where in each case $y$ runs through $A$.

By one half of Lavrentieff's theorem (Theorem 4.3.20 in Engelking's General Topology) we can find a $G_\delta$-set $G$ that contains~$A$ and a continuous map $f:G\to[0,1]$ that extends $g$. The complement of $G$ in $[0,1]$ is a countable union of closed sets, each of which is countable because they are closed and disjoint from the Bernstein set $A$. It follows that $f$ belongs to the family $\mathcal{F}$.

The arguments given above yield information about the locations of $s(x,0)$ and $s(x,1)$ whenever $x\in G$. Indeed, if $x\in G\setminus X$ then $(t\circ s)(x,0)=f(x)=(t\circ s)(x,1)$ and so in all cases $s(x,i)\in\{\langle f(x),0\rangle,\langle f(x),1\rangle\}$.

To summarize: divide $\mathbb{A}_X$ into two sets $P$, the points with first coordinate in $G$, and $Q$, the points with first coordinate outside $G$. The set $Q$ is countable and once we show that $f[G]$ is countable we can conclude that $s[\mathbb{A}_X]$ is countable and that $s$ is definitely not surjective.

Our first claim is that $E(f)$ is countable. We show this by showing that its intersection with the Bernstein set $A$ is countable.

Indeed, let $x\in E\cap A$, then $f(x)=x$ and hence $s(x)=\langle x,0\rangle$ or $s(x)=\langle x,1\rangle$; remember: the points of $A$ are split in $\mathbb{A}_Y$. So $E\cap A$ is the union of two sets $E_0=\{x\in E\cap A:s(x)=\langle x,0\rangle\}$ and $E_1=\{x\in E\cap A:s(x)=\langle x,1\rangle\}$.

By continuity of $s$ we have for every $x\in E_0$ an interval $(p_x,q_x)$ with rational end points that is mapped by $s$ into the interval $[0,\langle x,0\rangle]$ in $\mathbb{A}_Y$. It follows that if $x<y$ in $E_0$ then $y\in(p_y,q_y)\setminus (p_x,q_x)$. We see that $x\mapsto(p_x,q_x)$ is injective and $E_0$ is countable. Likewise $E_1$ is countable and we conclude that $E(f)\cap A$ is countable and hence so is $E(f)$ itself.

The second claim is that $f[S(f)]$ is countable. We show that $f[S(f)]$ is countable by showing that $f[C(f)\cap A]\cap V$ is countable. The contrapositive of the properties of our family of Bernstein sets then shows that $C(f)$ and hence $f[S(f)]$ is countable.

As above: let $x\in C(f)\cap A$ and assume $f(x)\in V$. Then $s(x)=\langle f(x),0\rangle$ or $s(x)=\langle f(x),1\rangle$. As above we split $C(f)\cap A$ into two pieces: $C_0=\{x:s(x)=\langle f(x),0\rangle\}$ and $C_1=\{x:s(x)=\langle f(x),1\rangle\}$.

If $x\in C_0$ then we take a rational interval $(p_x,q_x)$ that gets mapped into $[0,\langle f(x),0\rangle]$ by $s$. If $x,y\in C_0$ and $f(x)<f(y)$ then $y\in(p_y,q_y)\setminus (p_x,q_x)$, and we conclude that $x\mapsto(p_x,q_x)$ is injective and $C_0$ is countable. Likewise $C_1$ is countable.

It follows that $f[G]=E(f)\cup f[S(f)]$ is countable.

16-03-2023: I put a more leisurely explanation on ArXiV.org

Added 30-08-2024: it turns out that a forty-years old paper by Murray Bell contains all the ingredients to answer the original question a sequence $\langle B_n:2\le n<\omega\rangle$ of subalgebras of $\mathcal{P}(\omega)/\mathit{fin}$ such that $B_n$ is $\sigma$-$n$-linked but not $\sigma$-$(n+1)$-linked (for all $n$). This can be converted into a sequence as desired in the question. Details in the paper on ArXiV.org.

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