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babbys_first_hott [2015/02/06 13:58] nikolaj |
babbys_first_hott [2015/08/02 20:46] nikolaj |
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| {{ thed.jpg?X400}} | {{ thed.jpg?X400}} | ||
| - | We can inductively define the circle $\mathbb S$ as type by postulating | + | We can inductively define the circle $\mathbb S$ as type by postulating |
| $base:\mathbb S$ | $base:\mathbb S$ | ||
| - | |||
| and | and | ||
| - | |||
| $loop:base=_{\mathbb S}base$. | $loop:base=_{\mathbb S}base$. | ||
| - | |||
| Now | Now | ||
| - | |||
| $\dots,loop^{-2},loop^{-1},refl_{base},loop,loop^2,loop^3,\dots$ | $\dots,loop^{-2},loop^{-1},refl_{base},loop,loop^2,loop^3,\dots$ | ||
| - | |||
| are all terms of $base=_{\mathbb S}base$. | are all terms of $base=_{\mathbb S}base$. | ||
| This type can be shown to be equivalent (in the formal sense discussed below) to the integers. Indeed, recall that the first homotopy group of the circle is isomorphic to $\mathbb Z$. | This type can be shown to be equivalent (in the formal sense discussed below) to the integers. Indeed, recall that the first homotopy group of the circle is isomorphic to $\mathbb Z$. | ||
| Line 128: | Line 122: | ||
| $isSet(A):={\large{\prod}}_{x:A}{\large{\prod}}_{y:A}\,isProp(x=y)$ | $isSet(A):={\large{\prod}}_{x:A}{\large{\prod}}_{y:A}\,isProp(x=y)$ | ||
| - | The above says that $x$ connects to $y$ via at most one invertible path and hence this characterizes as set as a discrete category. Since sense, characterized like here as types with lack of other structure, are generally different from sets characterized in a theory like ZFC. | + | The above says that $x$ connects to $y$ via at most one invertible path and hence this characterizes as set as a discrete category. Since sets, characterized like here as types with lack of other structure, are generally different from sets characterized in a theory like ZFC. |
| == Homotopy theory == | == Homotopy theory == | ||
