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8 changes: 4 additions & 4 deletions book/C0-Preface.tex
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@ \chapter{Preface}\label{chapter.0}

This book is a work in progress --- including the acknowledgements below! Use at your own peril!

Categorical systems theory is an emerging field of mathematics which seeks to apply the methods of category theory to general systems theory. General systems theory is the study of systems --- ways things can be and change, and models thereof --- in full generality. The difficulty is that there doesn't seem to be a single core idea of what it means to be a ``system''. Different people have, for different purposes, come up with a vast array of different modeling techniques and definitions that could be called ``systems''. There is often little the same in the precise content of these definitions, though there are still strong, if informal, analogies to be made accross these different fields. This makes coming up with a mathematical theory of general systems tantalizing but difficult: what, after all, is a system in general?
Categorical systems theory is an emerging field of mathematics which seeks to apply the methods of category theory to general systems theory. General systems theory is the study of systems --- ways things can be and change, and models thereof --- in full generality. The difficulty is that there doesn't seem to be a single core idea of what it means to be a ``system''. Different people have, for different purposes, come up with a vast array of different modeling techniques and definitions that could be called ``systems''. There is often little the same in the precise content of these definitions, though there are still strong, if informal, analogies to be made across these different fields. This makes coming up with a mathematical theory of general systems tantalizing but difficult: what, after all, is a system in general?

Category theory has been describe as the mathematics of formal analogy making. It allows us to make analogies between fields by focusing not on content of the objects of those fields, but by the ways that the objects of those fields relate to one another. Categorical systems theory applies this idea to general systems theory, avoiding the issue of not having a contentful definition of system by instead focusing on the ways that systems interact with eachother and their environment.

Expand Down Expand Up @@ -41,7 +41,7 @@ \chapter{Preface}\label{chapter.0}
\end{itemize}
\end{informal}

We will give a semi-formal\footnote{And for experts, a formal definition, though we won't fully justify it.} definition of dynamical systems doctrine in \cref{Chapter.6}. For the first five chapters of this book on the other hand, we will work within a fixed doctrine of dynamical systems which we might call the \emph{parameter-setting} doctrine. This doctrine gives a particular answer to the above questions, based around the following defintion of a \emph{system}.
We will give a semi-formal\footnote{And for experts, a formal definition, though we won't fully justify it.} definition of dynamical systems doctrine in \cref{Chapter.6}. For the first five chapters of this book on the other hand, we will work within a fixed doctrine of dynamical systems which we might call the \emph{parameter-setting} doctrine. This doctrine gives a particular answer to the above questions, based around the following definition of a \emph{system}.

\begin{informal}\label{inf.dynam_sys}
A \emph{dynamical system} consists of:
Expand All @@ -54,7 +54,7 @@ \chapter{Preface}\label{chapter.0}
state that are \emph{exposed} or \emph{output} to the environment.
\end{informal}

In the first two chatpers, we will see a variety of examples of such systems, including discrete-time deterministic systems, systems of differential equations, and non-deterministic systems such as Markov decision processes. We will also see what composition patterns can be in the parameter-setting doctrine; they can be drawn as wiring diagrams like this:
In the first two chapters, we will see a variety of examples of such systems, including discrete-time deterministic systems, systems of differential equations, and non-deterministic systems such as Markov decision processes. We will also see what composition patterns can be in the parameter-setting doctrine; they can be drawn as wiring diagrams like this:
\[
\begin{tikzpicture}[oriented WD, every fit/.style={inner xsep=\bbx, inner ysep=\bby}, bb small]
\node[bb={2}{2}, fill=blue!10] (X1) {};
Expand Down Expand Up @@ -123,7 +123,7 @@ \chapter{Preface}\label{chapter.0}
it with the first one and then with the second one.
\item Suppose that we have a pair of
wiring patterns and compatible charts between them. If we
take a bunch of behaviors whose charts are compatable according to the first
take a bunch of behaviors whose charts are compatible according to the first
wiring pattern, wire them together, and then compose with a behavior of the
second chart, we get the same thing as if we compose them all with behaviors
of the first chart, noted that they were compatible with the second wiring
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38 changes: 19 additions & 19 deletions book/C1-.tex
Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,7 @@ \section{Introduction}\label{sec.chap1_intro}
\end{itemize}

Different people have decided on different answers to these questions for
different purposes. Here are three of the most widespread differents ways to answer those
different purposes. Here are three of the most widespread different ways to answer those
questions:
\begin{enumerate}
\item We'll assume the states form a discrete set, and that if we know the
Expand Down Expand Up @@ -113,7 +113,7 @@ \section{Introduction}\label{sec.chap1_intro}

The dynamical systems we will see in this book are \emph{open} in the sense that
they take in inputs from their environment and expose outputs back to their
environment. Because of this, our systems can interact with eachother. One
environment. Because of this, our systems can interact with each other. One
system can take what the other system outputs as part of its input, and the
other can take what the first outputs as part of its input. For example, when we
have a conversation, I take what I hear from you and use it to change how I
Expand Down Expand Up @@ -588,7 +588,7 @@ \subsection{Deterministic systems}\label{sec.deterministic_system}

\begin{example}[SIR model]\label{ex.SIR_model_discrete}
The set of states for a deterministic system doesn't need to be finite. The $\Sys{SIR}$ model
is an epimediological model used to study how a disease spreads through a
is an epidemiological model used to study how a disease spreads through a
population. ``SIR'' stands for ``susceptible'', ``infected'', and, rather
ominously, ``removed''. This model is usually presented as a system of
differential equations --- what we will call a differential system --- and we will see it in that form in \cref{ex.SIR_model_diff}.
Expand Down Expand Up @@ -714,7 +714,7 @@ \subsection{Deterministic systems}\label{sec.deterministic_system}
wiring together some systems.

For example, suppose we have an $\Sys{Agent}$ acting within a
$\Sys{Environment}$. The agent will take an action, and the evironment will
$\Sys{Environment}$. The agent will take an action, and the environment will
respond to that action. Depending on the action taken and response given, the
agent and the environment will update their states. We can model this by the
following wiring diagram:
Expand Down Expand Up @@ -756,8 +756,8 @@ \subsection{Differential systems}\label{sec.differential_system}
\emph{La nature ne fait jamais des sauts} - Liebniz
\end{quote}

A quirk of modeling dynamical systems as determinstic systems is that
determinstic systems lurch from one state to the next. In life, there are no
A quirk of modeling dynamical systems as deterministic systems is that
deterministic systems lurch from one state to the next. In life, there are no
next moments. Time, at least at human scales and to a first approximation, flows
continuously.

Expand Down Expand Up @@ -797,7 +797,7 @@ \subsection{Differential systems}\label{sec.differential_system}

The population of any predator will also change according to a birth rate
and death rate. Suppose we have a similarly defined system of $\Sys{Foxes}$
goverened whose population is governed by the differential equation
governed whose population is governed by the differential equation
\[
\frac{df}{dt} = \const{b}_{\Sys{Foxes}} \cdot f - \const{d}_{\Sys{Foxes}}
\cdot f.
Expand Down Expand Up @@ -904,13 +904,13 @@ \subsection{Differential systems}\label{sec.differential_system}
including the category $\Cat{Euc}$ of Euclidean spaces and smooth maps
(\cref{def.euc_cat}). It appears that a differential system is the same thing as a
deterministic system in the cartesian category $\Cat{Euc}$. But while the
$\rr^n$s occuring in $\update{S} : \rr^n \times \rr^m \to \rr^n$ look the
$\rr^n$s occurring in $\update{S} : \rr^n \times \rr^m \to \rr^n$ look the
same, they are in fact playing very different roles. The $\rr^n$ on the left
is playing the role of the state space, while the $\rr^n$ on the left is
playing the role of the tangent space at $s$ for some state $s \in \rr^n$. The
difference will be felt in \cref{chapter.3} when we study behaviors of
systems: the way a trajectory is defined is different
for differential systems and determinstic systems. For differential systems, a
for differential systems and deterministic systems. For differential systems, a
trajectory will be a solution to the system of differential equations, that
is, a function $s : \rr \to \rr^n$ which satisfies
$$\frac{ds}{dt}(t) = \update{S}(s(t), i(t)).$$
Expand All @@ -929,7 +929,7 @@ \subsection{Differential systems}\label{sec.differential_system}

\begin{example}\label{ex:lotka.volterra.model}
The system of $\Sys{Rabbits}$ has $1$ state variable (the population of
rabbits), $2$ parameters (the birth and death rates of the rabbbits), and $1$ exposed
rabbits), $2$ parameters (the birth and death rates of the rabbits), and $1$ exposed
variable. It exposes its whole state, so that $\expose{S} = \id$, and its
update is given by
\[
Expand Down Expand Up @@ -1470,7 +1470,7 @@ \subsection{Deterministic and differential systems as lenses}
input and following the arrow with that label, and then outputting the label on the
resulting node. When we wire together systems presented as transition diagrams,
the dynamics then involve reading the input labels of all inner systems, moving
along all the arrows with those labels, and then outputing the labels at each
along all the arrows with those labels, and then outputting the labels at each
state, possible into the input of another system.

\begin{exercise}\label{ex.wiring_transition_diagrams}
Expand Down Expand Up @@ -1579,12 +1579,12 @@ \subsection{Deterministic and differential systems as lenses}
populations can flow between cities.
\end{definition}

Now, to define a mutli-city $\Sys{SIR}$ model, we need to know what cities we
Now, to define a multi-city $\Sys{SIR}$ model, we need to know what cities we
are dealing with and how population flows between them. We'll call this a
\emph{population flow graph}.

\begin{definition}\label{def.population_flow_graph}
A \emph{population-flow graph} (for a mutli-city $\Sys{SIR}$ model) is a graph
A \emph{population-flow graph} (for a multi-city $\Sys{SIR}$ model) is a graph
whose nodes are labeled by cities and whose edges $\Sys{City}_1 \to
\Sys{City}_2$ are labeled by
$3 \times 3$ real diagonal matrices $\const{Flow}_{1\to
Expand Down Expand Up @@ -1986,7 +1986,7 @@ \subsection{Wiring diagrams as lenses in categories of arities}\label{sec:wiring
Wiring diagrams are designed to
express the flow of variables through the system; how they are to be copied from
one port to another, how they are to be shuffled about, and (though we haven't
had need for this yet) how they are to be deleted or forgotton.
had need for this yet) how they are to be deleted or forgotten.



Expand All @@ -1999,7 +1999,7 @@ \subsection{Wiring diagrams as lenses in categories of arities}\label{sec:wiring

\begin{definition}\label{defn:category.of.arities}
The category $\arity$ of arities is the free cartesian category generated by
a single object $\XX$. That is, $\arity$ constains an object $\XX$, called
a single object $\XX$. That is, $\arity$ contains an object $\XX$, called
the \emph{generic object}, and for any finite set $I$, there is an $I$-fold
power $\XX^I$ of $\XX$. The only maps are those that can be defined from the
product structure by pairing and projection.
Expand Down Expand Up @@ -2264,7 +2264,7 @@ \subsection{Wiring diagrams as lenses in categories of arities}\label{sec:wiring
\end{enumerate}
\end{exercise}

Ok, so the wiring diagrams correponds to the lenses in the category of arities.
Ok, so the wiring diagrams corresponds to the lenses in the category of arities.
But do they compose in the same way? Composition of wiring diagrams is given by
nesting: to compute $\lens{w^{\sharp}}{w} \then \lens{u^{\sharp}}{u}$, we fill
in the inner box of $\lens{u^{\sharp}}{u}$ with the outer box of
Expand Down Expand Up @@ -2623,7 +2623,7 @@ \subsection{Wiring diagrams as lenses in categories of arities}\label{sec:wiring
w &= (t : \Set{Hour},\, m : \Set{a.m./p.m.} \mapsto t : \Set{Hour},\, m : \Set{a.m./p.m.}) \\
w^{\sharp} &= (t : \Set{Hour},\, m : \Set{a.m./p.m.} \mapsto t : \Set{Hour})
\end{align*}
giving us a wiring diagram in $\Cat{WD}_{\cat{T}}$. We can then interepret
giving us a wiring diagram in $\Cat{WD}_{\cat{T}}$. We can then interpret
this as the lens from $\cref{ex.ClockWithDisplay}$ as the image of this wiring
diagram which interprets the types $\Set{a.m./p.m.}$ and $\Set{Hour}$ as the
actual sets $\{\const{a.m.},\const{p.m.}\}$ and $\{1, 2,\ldots, 12\}$ --- which is choosing the $C_{-} : \cat{T} \to \smset$ used in \cref{prop.interpret_typed_wiring_diagram}.
Expand Down Expand Up @@ -2779,7 +2779,7 @@ \subsection{Wiring diagrams with operations as lenses in Lawvere theories}\label

How do we know that the extra maps in a Lawvere theory really do come from the
operations of an algebraic theory? We show that the Lawvere theory satisfies a
certain universal property: cartesian functors out of it correpond to
certain universal property: cartesian functors out of it correspond to
\emph{models} of the theory. If this is the case, we say that the Lawvere theory
is \emph{presented} by the algebraic theory.

Expand Down Expand Up @@ -2845,7 +2845,7 @@ \subsection{Wiring diagrams with operations as lenses in Lawvere theories}\label
able to put matrices in the beads.
\end{example}

\section{Summary and Futher Reading}
\section{Summary and Further Reading}

In this first chapter, we introduced deterministic and differential systems and
saw how they could be composed using wiring diagrams. The trick is that both
Expand Down
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