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Diffstat (limited to 'provider/posts/simmons-intro-to-cat-t')
-rw-r--r-- | provider/posts/simmons-intro-to-cat-t/1.2.md | 45 |
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diff --git a/provider/posts/simmons-intro-to-cat-t/1.2.md b/provider/posts/simmons-intro-to-cat-t/1.2.md index 7759368..3e83b5f 100644 --- a/provider/posts/simmons-intro-to-cat-t/1.2.md +++ b/provider/posts/simmons-intro-to-cat-t/1.2.md | |||
@@ -68,3 +68,48 @@ Show that such objects and morphisms form a category $\ca{SetD}$ | |||
68 | $$(f \circ g)(\alpha) = f(b) = c$$ | 68 | $$(f \circ g)(\alpha) = f(b) = c$$ |
69 | </div> | 69 | </div> |
70 | </div> | 70 | </div> |
71 | |||
72 | <div class="exercise"> | ||
73 | Consider pairs $(A, \odot)$ where $A$ is a set and $\odot \subseteq A \times A$ is a binary relation on $A$. | ||
74 | |||
75 | Show that these pairs are objects of a category finding a sensible notion of morphism. | ||
76 | |||
77 | <div class="proof"> | ||
78 | For two such objects $(A, \odot), (B, \oplus)$ a morphism shall be a function $f : A \to B$ that respects the binary relation: | ||
79 | $$\forall a \odot a^\prime \ldotp f(a) \oplus f(a^\prime)$$ | ||
80 | We call the category comprised of such objects and arrows $\ca{RelH}$. | ||
81 | |||
82 | 1. For every $(A, \odot) \in \ca{RelH}$ there exists $\idarr{(A, \odot)}$ | ||
83 | |||
84 | $\id$ on $A$ is indeed a function which respects the binary relation ($\forall a \odot a^\prime \ldotp \id(a) \odot \id(a^\prime)$) and thus a morphism | ||
85 | |||
86 | 2. There exists a partial binary operation $\circ$ on the arrows of $\ca{RelH}$ | ||
87 | |||
88 | Given three objects $(A, \odot), (B, \oplus), (C, \otimes)$ and two functions $g: A \to B$ and $f: B \to C$ the function $f \circ g: A \to C$ is an arrow in $\ca{RelH}$, that is to say, $\forall a \odot a^\prime$: | ||
89 | $$a \odot a^\prime \implies g(a) \oplus g(a^\prime) \implies (g \circ f)(a) \otimes (g \circ f)(a^\prime)$$ | ||
90 | </div> | ||
91 | </div> | ||
92 | |||
93 | <div class="exercise"> | ||
94 | Topological spaces $(S, \mathcal{O}_S)$, where $S$ is a Set and $\mathcal{O}_S \subseteq \powerset(S)$, and continuous maps $f : S \to P$, that is $\forall V \in \mathcal{O}_T \ldotp f^\leftarrow(V) \in \mathcal{O}_S$, form a Category $\ca{Top}$. | ||
95 | |||
96 | 1. For every $(S, \mathcal{O}_S) \in \ca{Top}$ there exists $\idarr{(S, \mathcal{O}_S)}$ | ||
97 | <div class="proof"> | ||
98 | $\id$ on $S$ is indeed a continuous map and thus a morphism | ||
99 | </div> | ||
100 | |||
101 | 2. There exists a partial binary operation $\circ$ on the arrows of $\ca{Top}$ | ||
102 | <div class="proof"> | ||
103 | Given three spaces $(S, \mathcal{O}_S), (T, \mathcal{O}_T), (U, \mathcal{O}_U)$ and two continuous maps $g : S \to T$ and $f : T \to U$ the map $f \circ g : S \to U$ is an arrow in $\ca{Top}$, that is to say, it is contiuous: | ||
104 | $$V \in \mathcal{O}_U \implies g^\leftarrow(V) \in \mathcal{O}_T \implies (f \circ g)^\leftarrow(V) \in \mathcal{O}_S$$ | ||
105 | </div> | ||
106 | </div> | ||
107 | |||
108 | <div class="exercise"> | ||
109 | Let $A$ be an object of a category $\ca{C}$. | ||
110 | Show that $\hom{\ca{C}}{A}{A}$ is a monoid under composition. | ||
111 | |||
112 | <div class="proof"> | ||
113 | |||
114 | </div> | ||
115 | </div> | ||