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Expand Up @@ -99,10 +99,93 @@ theory that depend only on homotopy types.</p>
<ul>
<li><a href="#introduction" id="toc-introduction"><span
class="toc-section-number">1</span> Introduction</a></li>
<li><a href="#higher-covers" id="toc-higher-covers"><span
class="toc-section-number">2</span> Higher covers</a></li>
<li><a href="#where-the-curved-connection-is"
id="toc-where-the-curved-connection-is"><span
class="toc-section-number">3</span> Where the curved connection
is</a></li>
</ul>
</nav>
<h1 data-number="1" id="introduction"><span
class="header-section-number">1</span> Introduction</h1>
<blockquote>
<p>"It is always ourselves we work on, whether we realize it or not.
There is no other work to be done in the world." — Stephen Talbott,
<em>The Future Does Not Compute</em><span class="citation"
data-cites="talbott">(<a href="#ref-talbott"
role="doc-biblioref">Talbott, 1995</a>)</span></p>
</blockquote>
<p>Homotopy type theory offers us an opportunity to better understand
the mathematics of the past. It does this by moving more of the commonly
used structures into the foundational layer of mathematical logic, and
then asking us to express old ideas in the new foundations. The
difficulty of doing this varies considerably. Some would say that what
we gain is the ability to formalize the resulting math, but this makes
it appear that “banking theorems” is what math is. It’s not. Math is
about each person’s journey of understanding, which we share with each
other and at which we help each other succeed. The math is the means,
and the byproduct, of our own quest for understanding. I wanted to
understand geometry, connections, curvature, and gauge theory, and I
have done that. I’d like to share it with you.</p>
<p>Connections are not inherently infinitesimal. Infinitesimal
connections assign holonomy to curves if you integrate them, and if you
define holonomy directly on the space of paths in a manifold then you
can differentiate the assignment to get a 1-form with values in the Lie
algebra of the group where the holonomy takes it values (see <span
class="citation" data-cites="freed1992classical">(<a
href="#ref-freed1992classical" role="doc-biblioref">Freed,
1992</a>)</span> Appendix B).</p>
<p>Define type families.</p>
<p>Define identity types.</p>
<p>Define functions and functoriality.</p>
<p>Point out that functions are equipped with the full dimensionality of
effects, including on paths.</p>
<p>Define the delooping of a group.</p>
<p>Deconstruct a map to the delooping of a group.</p>
<h1 data-number="2" id="higher-covers"><span
class="header-section-number">2</span> Higher covers</h1>
<p>One problem with the space <span
class="math inline">\(\mathrm{BAut}_1(S^1)\)</span> is that it’s
spatially discrete. There are no smooth circles for smooth manifolds to
get hung up on. It has been argued that this means that any would-be
classifying map <span class="math inline">\(f:X\to
\mathrm{BAut}_1(S^1)\)</span> can only produce principal bundles with
<em>flat</em> connections. That would not achieve our goals, and so it
would be disappointing. It looks even worse when we notice that <span
class="math inline">\(f\)</span> must factor through the shape unit
<span
class="math inline">\((-)^\text{\textesh}:X\to\text{\textesh}X\)</span>
by the universal property of <span
class="math inline">\(\text{\textesh}\)</span> for a map into a discrete
space. And when we examine <span
class="math inline">\(\text{\textesh}X\)</span> we see that homotopic
paths are identified and so must apparently be sent to equal holonomies,
just as in a flat connection.</p>
<p>Fortunately there’s some hand-waving in the previous paragraph and
there are indeed non-flat connections in <span
class="math inline">\(\text{\textesh}f:\text{\textesh}X\to
\mathrm{BAut}_1(S^1)\)</span>. They are not the infinitesimal variety of
course. They are known as <em>discrete connections</em><span
class="citation" data-cites="crane_connections">(<a
href="#ref-crane_connections" role="doc-biblioref">Crane,
2010</a>)</span><span class="citation" data-cites="crane_ddg">(<a
href="#ref-crane_ddg" role="doc-biblioref">Crane, 2020</a>)</span>.
Discrete connections are an assignment of group elements to discrete
paths in a combinatorial manifold. If we represent the homotopy type of
<span class="math inline">\(X\)</span> by a discrete combinatorial
manifold, then we can observe a meaningful discrete curvature of the
principal bundle, and in so doing access the homotopy-invariant aspects
of gauge theory.</p>
<p>What makes this all work is that <span
class="math inline">\(S^1\)</span> is a discrete 1-type, not a discrete
set. A map from <span class="math inline">\(\text{\textesh}X\to
BG\)</span> for a discrete set-level group <span
class="math inline">\(G\)</span> would indeed carry its unique
<em>flat</em> connection, since homotopic paths must map to the same set
element. But so long as we’re willing to work with discrete spaces and
discrete connections, we can see geometry inside homotopy type
theory!</p>
<p>Following David Jaz Myers in <span class="citation"
data-cites="myersgood">(<a href="#ref-myersgood"
role="doc-biblioref">Myers, 2022</a>)</span>, we define a cover as
Expand Down Expand Up @@ -164,49 +247,129 @@ class="math inline">\(B\)</span> is equivalent to <span
class="math inline">\(\text{\textesh}B\to
\mathsf{Type}_{\text{\textesh}{}_{1}}\)</span>.</em></p>
</div>
<p>This is the type we will examine.</p>
<p>Let <span class="math inline">\(S^1\)</span> be the homotopical
circle. This is a 1-type and its loop space is <span
class="math inline">\(\ensuremath{\mathbb{Z}}\)</span>. We can deloop
<span class="math inline">\(S^1\)</span> by forming its type of torsors.
This is equivalent to <span class="math inline">\(\mathrm{BAut}_1
S^1\stackrel{\textup{def}}{=}\sum_{(X:\mathsf{Type})}||X=S^1||_0\)</span>.
<span class="math inline">\(\mathrm{BAut}_1(S^1)\)</span> is a 2-type.
(see <span class="citation" data-cites="buchholtz2023central">(<a
<p>This is the type we will examine: maps from discrete <span
class="math inline">\(n\)</span>-types to the type of discrete
1-types.</p>
<p>For example, let <span class="math inline">\(S^1\)</span> be the
higher inductive type generated by</p>
<ul>
<li><p><span class="math inline">\(\mathsf{base}:S^1\)</span></p></li>
<li><p><span
class="math inline">\(\mathsf{loop}:\mathsf{base}=\mathsf{base}\)</span></p></li>
</ul>
<p>We can deloop <span class="math inline">\(S^1\)</span> by forming its
type of torsors. This is equivalent to <span
class="math inline">\(\mathrm{BAut}_1(S^1)\stackrel{\textup{def}}{=}\sum_{(X:\mathsf{Type})}||X=S^1||_0\)</span>,
the type of pairs of a type together with an equivalence class of
isomorphisms with <span class="math inline">\(S^1\)</span> (see <span
class="citation" data-cites="buchholtz2023central">(<a
href="#ref-buchholtz2023central" role="doc-biblioref">Buchholtz <em>et
al.</em>, 2023</a>)</span>).</p>
<p>Since <span
class="math inline">\(S^1:\mathsf{Type}_{\text{\textesh}{}_{1}}\)</span>
we see that <span class="math inline">\(\mathrm{BAut}_1(S^1)\)</span> is
a subtype of <span
class="math inline">\(\mathsf{Type}_{\text{\textesh}{}_{1}}\)</span>.
Which is to say, among the type of 2-covers of <span
class="math inline">\(B\)</span> are those whose fibers are all <span
class="math inline">\(S^1\)</span>-torsors.</p>
<p>If we believe in the shape operator, then we can design our own
discrete types that implement known combinatorial versions of spaces,
and discard the original smooth and continuous spaces completely. Mike
Shulman proves in <span class="citation"
data-cites="shulman_cohesion">(<a href="#ref-shulman_cohesion"
role="doc-biblioref">Shulman, 2017</a>)</span> that the shapes of the
smooth circle and sphere are <span class="math inline">\(S^1\)</span>
and <span class="math inline">\(S^2\)</span> respectively.</p>
<p>One compelling candidate for a general type of combinatorial spaces
are <em>higher polytopes</em>. Polytopes are simply posets satisfying
some extra conditions that make them a generalization of polyhedrons to
arbitrary dimension. The poset is just the information about the set of
vertices, edges, faces, and higher faces and their containment. By using
the higher-dimensional constructors of higher inductive types, we can
import a polytope into homotopy type theory as a discrete type that
intuitively captures the homotopy type of any manifolds we are
interested in!</p>
<p>The usual homotopical circle and sphere are not polytopes because the
edges don’t have enough endpoints... [make the cube and square
example].</p>
<p>If <span class="math inline">\(f\)</span> then <span
class="math display">\[F\]</span></p>
<div class="mylemma">
<p><strong>Lemma 3</strong>. <em>Terms of <span
class="math inline">\(\mathrm{BAut}_1(S^1)\)</span> are discrete
1-types.</em></p>
</div>
<div class="proof">
<p><em>Proof.</em> TBD. ◻</p>
</div>
<p>Consider a generalization of <span
class="math inline">\(S^1\)</span>, an <em><span
class="math inline">\(n\)</span>-gon</em> <span
class="math inline">\(C_n\)</span>, generated by</p>
<ul>
<li><p><span class="math inline">\(v_1,\ldots,v_n:C_n\)</span></p></li>
<li><p><span class="math inline">\(e_1:v_1=v_2\)</span></p></li>
<li><p><span class="math inline">\(e_2:v_2=v_3\)</span></p></li>
<li><p><span class="math inline">\(\ldots\)</span></p></li>
<li><p><span class="math inline">\(e_{n-1}:v_{n-1}=v_n\)</span></p></li>
<li><p><span class="math inline">\(e_n:v_n=v_1\)</span></p></li>
</ul>
<div class="mylemma">
<p><strong>Lemma 4</strong>. <em>We have <span
class="math inline">\(g:\prod_{(n:\ensuremath{\mathbb{N}})}C_n\simeq
S^1\)</span></em></p>
</div>
<p>Generalize <span class="citation" data-cites="hottbook">(<a
href="#ref-hottbook" role="doc-biblioref">Univalent Foundations Program,
2013</a>)</span> Lemma 6.5.1.0◻</p>
<p><span class="math inline">\(n\)</span>-gons are all discrete
1-types:</p>
<div class="mydef">
<p><strong>Definition 3</strong>. <em>Let <span
class="math inline">\(c:\ensuremath{\mathbb{N}}\to
\mathsf{Type}_{\text{\textesh}{}_{1}}\)</span> be given by <span
class="math inline">\(c(n)=(C_n, ||g(n)||_0)\)</span>.</em></p>
</div>
<p>The reason we’re interested in these is that they form arbitrarily
fine-grained approximations to the smooth circle. We can consider an
<span class="math inline">\(n\)</span>-gon to be a circle that has a
notion of “going around <span class="math inline">\(1/n\)</span>th of
the way”.</p>
<p>Now let’s re-point <span
class="math inline">\(\mathrm{BAut}_1(S^1)\)</span> at <span
class="math inline">\(C_4\)</span>. We can use a + to denote a pointed
type, and we can decorate it with the base point, like so: <span
class="math inline">\({\mathrm{BAut}_1(S^1)}_{+{C_4}}\)</span>. We can
then refer to the underlying type (first projection from the pointed
type) with <span class="math inline">\(\mathrm{BAut}_1(S^1)_-\)</span>.
This is similar to the notation <span
class="math inline">\(\mathrm{BAut}_1(S^1)_{\div}\)</span> you find in
the Symmetry book <span class="citation" data-cites="Symmetry">(<a
href="#ref-Symmetry" role="doc-biblioref">Bezem <em>et al.</em>,
2023</a>)</span>.</p>
<h1 data-number="3" id="where-the-curved-connection-is"><span
class="header-section-number">3</span> Where the curved connection
is</h1>
<p>Let <span
class="math inline">\({B}_{+{b}}:\mathsf{Type}_{\text{\textesh}*}\)</span>
be a pointed discrete type and let <span
class="math inline">\(f:{B}_{+{b}}\,\cdot\!\to{\mathrm{BAut}_1(S^1)}_{+{C_4}}\)</span>.
If we need to reference the proof of pointedness we’ll call it <span
class="math inline">\(*_f:f(b)=C_4\)</span>.</p>
<div class="mylemma">
<p><strong>Lemma 5</strong>. <em><span
class="math inline">\(\text{\textesh}(X\to Y)\simeq \text{\textesh}X\to
\text{\textesh}Y\)</span></em></p>
</div>
<p>.</p>
<div class="proof">
<p><em>Proof.</em> TODO ◻</p>
</div>
<p>And so any results that depend only on the homotopy type of, say, a
classifying map, can be studied in HoTT by replacing both the domain and
the classifying type with their homotopy types.</p>
<div class="myprop">
<p><strong>Proposition 1</strong>. <em>(<span class="citation"
data-cites="atiyah1983yang">(<a href="#ref-atiyah1983yang"
role="doc-biblioref">Atiyah & Bott, 1983</a>)</span> Proposition
2.4) Let <span class="math inline">\(BG\)</span> be the classifying
space for a compact Lie group <span class="math inline">\(G\)</span>.
Then in homotopy theory <span
class="math display">\[B\mathscr{G}(P)=\mathrm{Map}_P(M, BG)\]</span>
where <span class="math inline">\(\mathscr{G}(P)\)</span> is the group
of automorphisms of the principal bundle <span
class="math inline">\(P\)</span>, and where <span
class="math inline">\(\mathrm{Map}_P\)</span> denotes the connected
component of <span class="math inline">\(M\to BG\)</span> containing the
classifying map of <span class="math inline">\(P\)</span>.</em></p>
</div>
<div id="refs" class="references csl-bib-body hanging-indent"
data-entry-spacing="0" role="list">
<div id="ref-atiyah1983yang" class="csl-entry" role="listitem">
<span class="smallcaps">Atiyah</span>, M.F. & <span
class="smallcaps">Bott</span>, R. (1983) The yang-mills equations over
riemann surfaces. <em>Philosophical Transactions of the Royal Society of
London. Series A, Mathematical and Physical Sciences</em>,
<strong>308</strong>, 523–615.
</div>
<div id="ref-Symmetry" class="csl-entry" role="listitem">
<span class="smallcaps">Bezem</span>, M., <span
class="smallcaps">Buchholtz</span>, U., <span
class="smallcaps">Cagne</span>, P., <span
class="smallcaps">Dundas</span>, B.I., & <span
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</div>
<div id="ref-buchholtz2023central" class="csl-entry" role="listitem">
<span class="smallcaps">Buchholtz</span>, U., <span
class="smallcaps">Christensen</span>, J.D., <span
Expand All @@ -215,15 +378,38 @@ class="smallcaps">Rijke</span>, E. (2023) <a
href="https://arxiv.org/abs/2301.02636">Central h-spaces and banded
types</a>.
</div>
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& Associates.
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<span class="smallcaps">Univalent Foundations Program</span> (2013)
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Institute for Advanced Study: <a
href="https://homotopytypetheory.org/book"
class="uri">https://homotopytypetheory.org/book</a>.
</div>
</div>
</body>
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