# How to formalize in terms of category theory?

We define a recursive map as maps, $\chi \to \xi^{'}, \, \chi^{'} \to \xi^{''}, \, \chi^{''} \to \xi^{'''}, \ldots, \chi^{n} \to \xi^{n+1} \wedge \xi \to \chi, \, \xi^{'} \to \chi{'}, \xi^{''} \to \chi^{''}, \ldots \xi^{n} \to \chi^{n}$. Explicitly the maps are $\psi_1 : \chi^{n} \to \xi^{n+1}$ and $\psi_2 : \xi^{n} \to \chi^{n}$ between objects, $\chi$ and $\xi$, such that the induced maps $\chi \to \chi^{k}$ and $\xi \to \xi^{k}$ have the property that, $\forall j \leq k$, $\chi \simeq \chi^{j}$ and $\xi \simeq \xi^{j}$.

$$\chi \underset{\psi_1,\psi_2}{\overset{k}{\circlearrowright}} \xi := \chi \overset{\psi_1}{\to} \xi^{'} \overset{\psi_2}{\to} \chi^{'} \overset{\psi_1}{\to} \xi^{''} \overset{\psi_2}{\to} \cdots \overset{\psi_1}{\to} \xi^{k} \overset{\psi_2}{\to} \chi^{k}$$

$$\chi \underset{\psi^{-1}_2,\psi^{-1}_1}{\overset{k}{\circlearrowleft}} \xi := \chi^{k} \overset{\psi^{-1}_2}{\to} \xi^{k} \overset{\psi^{-1}_1}{\to} \cdots \overset{\psi^{-1}_2}{\to} \xi^{''} \overset{\psi^{-1}_1}{\to} \chi^{'} \overset{\psi^{-1}_2}{\to} \xi^{'} \overset{\psi^{-1}_1}{\to} \chi$$ and

Thus, our notation for a recursive map requires two maps $\psi_1, \, \psi_2$, two objects on which these maps are defined, as well as the number of recursions, $k$

This was written in an old paper I came across, and I'd like to see how it can be formalized using category theory, as the relation seems relatively transparent.

If it helps, the laymans example given is between an interval, $\chi$ and a square, $\xi$. So given an interval $[0,x_0]$, we map into a square by $[0,x_0+a] \times [0,x_0+a]$, then back to an interval as $[0,x_0+a -b]$ and once again back into the square by $[0,x_0+2a-b] \times [0,x_0+2a-b]$ and back into the interval as $[0,x_0+2a-2b]$, $k$ times. Clearly $\chi \simeq \chi^j$ and $\xi \simeq \xi^j$ So here I would think loosely the categories are the squares and the intervals, with the defined morphisms...The author intends $\simeq$ to be homotopic, but I don't believe this is necessary, I believe a more general categorical equivalence could be suitable.

I would imagine it would go a little something like this:

Let $\mathscr{C}$ category with a class of objects $\mathscr{A} := \{ A, A^{'}, A^{''}, \ldots , A^n \}$ and $\mathscr{B} := \{ B, B^{'}, B^{''}, \ldots, B^{n} \}$ with morphisms $a := \{ a_1, a_2, \ldots , a_n \}$ and $b := \{ b_1, b_2, \ldots, b_n \}$ where $a_i : A^{i} \to B^{i+1}$ and $b_i : A^{i} \to B^{i}$ with the condition that $A \simeq A^{j}$ and $B \simeq B^{j}$, for some equivalence relation $\simeq$. We have the condition that all compositions $a \circ b$ and $b \circ a$ represent this equivalence relation, for example, a homeomorphism.

Thus we define our recursive map as
$$\mathscr{A} \underset{a,b}{\overset{k}{\circlearrowright}} \mathscr{B} := A \overset{a_1}{\to} B^{'} \overset{b_1}{\to} A^{'} \overset{a_2}{\to} B^{''} \overset{b_2}{\to} \cdots \overset{a_{k-1}}{\to} B^{k} \overset{b_k}{\to} A^{k}$$ with inverse

$$\mathscr{A} \underset{a^{-1}, b^{-1}}{\overset{k}{\circlearrowleft}} \mathscr{B} := A^{k} \overset{b^{-1}_k}{\to} B^{k} \overset{a^{-1}_{k-1}}{\to} \cdots \overset{b^{-1}_2}{\to} B^{''} \overset{a^{-1}_2}{\to} A^{'} \overset{b^{-1}_1}{\to} B^{'} \overset{a^{-1}_1}{\to} A$$

This is where I'm confused in terms of defintion. How to define this categorically, properly?

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Let $\overline{A}$ be the diagram with objects $A,A',A'', \cdots$, with a unique isomorphism between each pair of objects (in the example you give, this would be the scaling map - if you want a more general homotopy equivalence I suppose you would have to move to some sort of homotopy category to make the inverses work out right), and let $\overline{B}$ be the diagram whose objects are $B,B',B'', \cdots$ with similarly defined arrows.
Then, a recursive map is just a natural transformation from $\overline{A}$ to $\overline{B}$ along with a natural transformation $\overline{B} \to \overline{A}$ such that the composition $\overline{A} \to \overline{B} \to \overline{A}$ is one of the arrows in the diagram.
A diagram of shape $I$ in $C$ is a functor from $I$ to $C$, but that's not very enlightening. Essentially what it means in this case is that you're considering objects (e.g. all the A,A',A'', ....), which in your case are topological spaces, and "arrows" between the object, which correspond to functions (the same way we have "objects" and "arrows/morphisms" in category theory. (EDIT: Sorry, hit enter too early) A natural transformation between the two diagrams is defined at en.wikipedia.org/wiki/Natural_transformation, which then means that if we compose the maps $A \to A'$ –  Colin Aitken Mar 11 at 4:09
and $A' \to B''$ we get the same function we would get if we composed $A \to B'$ and $B' \to B'',$ as well as the fact that if we transform $A \to A'$ under the transformation we get $B' \to B''$, and vice versa. –  Colin Aitken Mar 11 at 4:13