I'll start by writing the answers to my questions. The answers to **MD 2** and **MD 3** are given distinctly by:

$$

\vee \dashv \Delta \dashv \wedge

$$

In **MD 4** I ask about the special case of \\(\mathbb{N}\\) ordered by the *evenly divides* relation \\(\cdot\ |\ \cdot\\), and the answer is

$$

lcm \dashv \Delta \dashv gcd

$$

As I was saying, the poset \\((\mathbb{N}, \cdot\ |\ \cdot)\\) does not have infima or suprema, so you can't use them directly to figure all of this out.

It's often nice to operate as if we have a infima and suprema for a preorder \\((P,\leq_P)\\) even if it doesn't have them. Also, it would be nice if it was a poset!

We can have all of this by constructing the smallest poset that has them and embeds \\(P\\). It is called the [*Dedekind–MacNeille completion*](https://en.wikipedia.org/wiki/Dedekind%E2%80%93MacNeille_completion) of \\(P\\). It is related to the Dedekind cut construction of the real numbers.

Dedekind–MacNeille gives rise to a *monad* \\(\mathbf{DM}\\) on the category of preorders with monotone maps as morphisms.

> **Definition.** For a given preorder \\((P,\leq_P)\\), let

>

> - \\(A^u := \\{p \in P\ :\ \forall a \in A. a \leq p\\}\\) and

> - \\(A^d := \\{p \in P\ :\ \forall a \in A. p \leq a\\}\\)

>

> Define \\(\mathbf{DM}(P) := \\{A \subseteq P\ :\ A = (A^u)^d\\}\\).

>

> The structure \\((\mathbf{DM}(P), \subseteq, \bigcup, \bigcap)\\) is the Dedekind–MacNeille completion of \\(P\\).

>

> The *principle ideal* function \\((\cdot \downarrow) : P \to \mathbf{DM}(P)\\) takes every element to its completion \\(x \downarrow\;:= \\{x\\}^d\\).

>

> Finally, we can lift every function \\(f: A \to B\\) between two posets \\(A\\) and \\(B\\) into a function between their completions \\(f^{\mathbf{DM}} : \mathbf{DM}(A) \to \mathbf{DM}(B)\\) using:

> $$ f^{\mathbf{DM}}(X) := ((f_!(X))^u)^d $$

By convention its nice to distinguish objects in the completed structures with the Fracktur font \\(\mathfrak{a}, \mathfrak{b}, \ldots\\)

> **Lemma.** Let \\(f: A \to B\\) and \\(g: B \to A\\) be maps on the preorders \\(A\\) and \\(B\\). Then:

>

> $$ \begin{eqnarray} f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} & \Longleftrightarrow & \forall \mathfrak{b}. g^{\mathbf{DM}}(\mathfrak{b}) = \bigcup\{ \mathfrak{a} \in \mathbf{DM}(A)\ :\ f^{\mathbf{DM}}(\mathfrak{a}) \subseteq \mathfrak{b} \} \\ & \Longleftrightarrow & \forall \mathfrak{a}. f^{\mathbf{DM}}(\mathfrak{a}) = \bigcap\{ \mathfrak{b} \in \mathbf{DM}(B)\ :\ \mathfrak{b} \subseteq g^{\mathbf{DM}}(\mathfrak{a}) \} \end{eqnarray} $$

> and

> $$ f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} \Longrightarrow f \dashv g $$

**Proof.** \\(f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} \Longrightarrow f \dashv g\\) follows by naturality of the principle ideal operation \\((\cdot\downarrow)\\). See [Davey and Priestley (2002), §7.38 The Dedekind–MacNeille completion](https://books.google.com/books?id=vVVTxeuiyvQC&pg=PA166#v=onepage&q&f=false). \\(\Box\\)

So certainly proving a Galois adjunction in the Dedekind–MacNeille completion *suffices* to show a Galois connection.

I think the converse of this Lemma is true too but I can't find a reference:

> **Conjecture**. \\(f \dashv g \Longrightarrow f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} \\)

This would give a full on *transfer theorem*.

If I find the time I will tackle this, but I also wanted to do some Haskell in another thread today, so I might not get around to it until the weekend.

$$

\vee \dashv \Delta \dashv \wedge

$$

In **MD 4** I ask about the special case of \\(\mathbb{N}\\) ordered by the *evenly divides* relation \\(\cdot\ |\ \cdot\\), and the answer is

$$

lcm \dashv \Delta \dashv gcd

$$

As I was saying, the poset \\((\mathbb{N}, \cdot\ |\ \cdot)\\) does not have infima or suprema, so you can't use them directly to figure all of this out.

It's often nice to operate as if we have a infima and suprema for a preorder \\((P,\leq_P)\\) even if it doesn't have them. Also, it would be nice if it was a poset!

We can have all of this by constructing the smallest poset that has them and embeds \\(P\\). It is called the [*Dedekind–MacNeille completion*](https://en.wikipedia.org/wiki/Dedekind%E2%80%93MacNeille_completion) of \\(P\\). It is related to the Dedekind cut construction of the real numbers.

Dedekind–MacNeille gives rise to a *monad* \\(\mathbf{DM}\\) on the category of preorders with monotone maps as morphisms.

> **Definition.** For a given preorder \\((P,\leq_P)\\), let

>

> - \\(A^u := \\{p \in P\ :\ \forall a \in A. a \leq p\\}\\) and

> - \\(A^d := \\{p \in P\ :\ \forall a \in A. p \leq a\\}\\)

>

> Define \\(\mathbf{DM}(P) := \\{A \subseteq P\ :\ A = (A^u)^d\\}\\).

>

> The structure \\((\mathbf{DM}(P), \subseteq, \bigcup, \bigcap)\\) is the Dedekind–MacNeille completion of \\(P\\).

>

> The *principle ideal* function \\((\cdot \downarrow) : P \to \mathbf{DM}(P)\\) takes every element to its completion \\(x \downarrow\;:= \\{x\\}^d\\).

>

> Finally, we can lift every function \\(f: A \to B\\) between two posets \\(A\\) and \\(B\\) into a function between their completions \\(f^{\mathbf{DM}} : \mathbf{DM}(A) \to \mathbf{DM}(B)\\) using:

> $$ f^{\mathbf{DM}}(X) := ((f_!(X))^u)^d $$

By convention its nice to distinguish objects in the completed structures with the Fracktur font \\(\mathfrak{a}, \mathfrak{b}, \ldots\\)

> **Lemma.** Let \\(f: A \to B\\) and \\(g: B \to A\\) be maps on the preorders \\(A\\) and \\(B\\). Then:

>

> $$ \begin{eqnarray} f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} & \Longleftrightarrow & \forall \mathfrak{b}. g^{\mathbf{DM}}(\mathfrak{b}) = \bigcup\{ \mathfrak{a} \in \mathbf{DM}(A)\ :\ f^{\mathbf{DM}}(\mathfrak{a}) \subseteq \mathfrak{b} \} \\ & \Longleftrightarrow & \forall \mathfrak{a}. f^{\mathbf{DM}}(\mathfrak{a}) = \bigcap\{ \mathfrak{b} \in \mathbf{DM}(B)\ :\ \mathfrak{b} \subseteq g^{\mathbf{DM}}(\mathfrak{a}) \} \end{eqnarray} $$

> and

> $$ f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} \Longrightarrow f \dashv g $$

**Proof.** \\(f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} \Longrightarrow f \dashv g\\) follows by naturality of the principle ideal operation \\((\cdot\downarrow)\\). See [Davey and Priestley (2002), §7.38 The Dedekind–MacNeille completion](https://books.google.com/books?id=vVVTxeuiyvQC&pg=PA166#v=onepage&q&f=false). \\(\Box\\)

So certainly proving a Galois adjunction in the Dedekind–MacNeille completion *suffices* to show a Galois connection.

I think the converse of this Lemma is true too but I can't find a reference:

> **Conjecture**. \\(f \dashv g \Longrightarrow f^{\mathbf{DM}} \dashv g^{\mathbf{DM}} \\)

This would give a full on *transfer theorem*.

If I find the time I will tackle this, but I also wanted to do some Haskell in another thread today, so I might not get around to it until the weekend.