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% This file: (find-LATEX "2020J-ops-new.tex")
% Newer version: (find-LATEX "2021planar-HAs-2.tex")
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% (defun c () (interactive) (find-LATEXsh "lualatex -record 2020J-ops-new.tex" :end))
% (defun d () (interactive) (find-pdf-page "~/LATEX/2020J-ops-new.pdf"))
% (defun d () (interactive) (find-pdftools-page "~/LATEX/2020J-ops-new.pdf"))
% (defun e () (interactive) (find-LATEX "2020J-ops-new.tex"))
% (defun u () (interactive) (find-latex-upload-links "2020J-ops-new"))
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% (find-LATEX "2019.mk")
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% Based on: (find-LATEX "2019J-ops.tex")
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\documentclass[oneside]{article}
\usepackage[colorlinks,citecolor=DarkRed,urlcolor=DarkRed]{hyperref} % (find-es "tex" "hyperref")
\usepackage{tocloft} % (find-es "tex" "tocloft")
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\xyoption{curve} % For the ".curve=" feature in 2D diagrams
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% (find-es "tex" "geometry")
\begin{document}
\input 2017planar-has-defs.tex % (find-LATEX "2017planar-has-defs.tex")
\input 2019J-ops-defs.tex % (find-LATEX "2019J-ops-defs.tex")
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% %L dofile "edrxpict.lua" -- (find-LATEX "edrxpict.lua")
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\def\ovl{\overline}
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% (find-LATEX "idarct/idarct-preprint.tex")
% «title» (to ".title")
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\title{Planar Heyting Algebras for Children 2: Local Operators, J-Operators, and Slashings}
\author{Eduardo Ochs}
\maketitle
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% «abstract» (to ".abstract")
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\begin{abstract}
Choose a topos $\calE$. There are several different ``notions of
sheafness'' on $\calE$. How do we visualize them?
Let's refer to the classifier object of $\calE$ as $Ω$, and to its
Heyting Algebra of truth-values, $\Sub(1_\calE)$, as $H$; we will
sometimes call $H$ the ``logic'' of the topos. There is a well-known
way of representing notions of sheafness as morphisms $j:Ω→Ω$, but
these `$j$'s yield big diagrams when we draw them explicitly; here we
will see a way to represent these `$j$'s as maps $J:H→H$ in a way that
is much more manageable.
In the previous paper of this series --- called \cite{OchsPH1} from
here on --- we showed how certain toy models of Heyting Algebras,
called ``ZHAs'', can be used to develop visual intuition for how
Heyting Algebras and Intuitionistic Propositional Logic work; here we
will extend that to sheaves. The full idea is this: {\sl notions of
sheafness} correspond to {\sl local operators} and vice-versa; {\sl
local operators} correspond to {\sl J-operators} and vice-versa; if
our Heyting Algebra $H$ is a ZHA then {\sl J-operators} correspond to
{\sl slashings} on $H$, and vice-versa; {\sl slashings} on $H$
correspond to {\sl ``sets of question marks''} and vice-versa, and
each set of question marks induces a notion of {\sl erasing and
reconstructing}, which induces a sheaf. Also, every ZHA $H$
corresponds to an {(acyclic) 2-column graph}, and vice-versa, and for
any two-column graph $(P,A)$ the logic of the topos $\Set^{(P,A)}$ is
exactly the ZHA $H$ associated to $(P,A)$.
The introduction of \cite{OchsPH1} discusses two different senses in
which a mathematical text can be ``for children''. The first sense
involves some precise metamathetical tools for transfering knowledge
back and forth between a general case ``for adults'' and a toy model
``for children''; the second sense is simply that the text's
presentation has few prerequisites and never becomes too abstract.
Here we will use the second sense: everything here, except for the
last section, should be accessible to students who have taken a course
on Discrete Mathematics and read \cite{OchsPH1}. This means that
categories, toposes, sheaves and the maps $j:Ω→Ω$ only appear in the
last section, and before that we deal only with the J-operators
$J:H→H$, how they correspond to slashings and sets of question marks,
and how they form an algebra.
\end{abstract}
% (find-books "__cats/__cats.el" "bell")
% (find-belltpage (+ 14 163) "modality")
% (find-belltpage (+ 14 164) "universal closure operation")
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% (find-books "__cats/__cats.el" "johnstone-elephant")
% (find-elephantpage (+ 17 195) "A4.4 Local Operators")
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% (find-books "__cats/__cats.el" "fourman-scott")
% (find-slnm0753page (+ 16 324) "J-operators")
% TODO: add this (and an abstract):
% (ph2p 4 "piccs-and-slashings")
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% «abstract-arxiv» (to ".abstract-arxiv")
%
% Choose a topos $E$. There are several different "notions of
% sheafness" on $E$. How do we visualize them?
%
% Let's refer to the classifier object of $E$ as $\Omega$, and to its
% Heyting Algebra of truth-values, $Sub(1_E)$, as $H$; we will
% sometimes call $H$ the "logic" of the topos. There is a well-known
% way of representing notions of sheafness as morphisms $j:\Omega\to
% \Omega$, but these `$j$'s yield big diagrams when we draw them
% explicitly; here we will see a way to represent these `$j$'s as maps
% $J:H\to H$ in a way that is much more manageable.
%
% In the previous paper of this series we showed how certain toy
% models of Heyting Algebras, called "ZHAs", can be used to develop
% visual intuition for how Heyting Algebras and Intuitionistic
% Propositional Logic work; here we will extend that to sheaves. The
% full idea is this: notions of sheafness correspond to local
% operators and vice-versa; local operators correspond to J-operators
% and vice-versa; if our Heyting Algebra $H$ is a ZHA then J-operators
% correspond to slashings on $H$, and vice-versa; slashings on $H$
% correspond to "sets of question marks" and vice-versa, and each set
% of question marks induces a notion of erasing and reconstructing,
% which induces a sheaf. Also, every ZHA $H$ corresponds to an
% (acyclic) 2-column graph, and vice-versa, and for any two-column
% graph $(P,A)$ the logic of the topos $\mathbf{Set}^{(P,A)}$ is
% exactly the ZHA $H$ associated to $(P,A)$.
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% «toc» (to ".toc")
% (jonp 2 "toc")
% (jona "toc")
% (favp 2 "toc")
% (fav "toc")
% (find-es "tex" "tocloft")
\renewcommand{\cfttoctitlefont}{\bfseries}
\setlength{\cftbeforesecskip}{2.5pt}
\tableofcontents
% \end{document}
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% «parts» (to ".parts")
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\newpage
\input 2019J-ops-slashings.tex % (find-LATEX "2019J-ops-slashings.tex")
\newpage
\input 2019J-ops-logic.tex % (find-LATEX "2019J-ops-logic.tex")
\newpage
\input 2019J-ops-midway.tex % (find-LATEX "2019J-ops-midway.tex")
\newpage
\input 2019J-ops-cubes.tex % (find-LATEX "2019J-ops-cubes.tex")
\newpage
\input 2019J-ops-valuations.tex % (find-LATEX "2019J-ops-valuations.tex")
\newpage
\input 2019J-ops-algebra.tex % (find-LATEX "2019J-ops-algebra.tex")
\newpage
\input 2019J-ops-categories.tex % (find-LATEX "2019J-ops-categories.tex")
\newpage
\input 2019J-ops-classifier.tex % (find-LATEX "2019J-ops-classifier.tex")
\newpage
\input 2019J-ops-kan.tex % (find-LATEX "2019J-ops-kan.tex")
%L write_dnt_file()
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\newpage
\printbibliography
\end{document}
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% <make>
* (eepitch-shell)
* (eepitch-kill)
* (eepitch-shell)
# (find-LATEXfile "2019planar-has-1.mk")
make -f 2019.mk STEM=2020J-ops-new veryclean
make -f 2019.mk STEM=2020J-ops-new pdf
% Local Variables:
% coding: utf-8-unix
% ee-tla: "jon"
% End: