Topos with $\Omega = [0,1]$? – Analysis and Examples

lo.logicpr.probabilitytopos-theory

For weakly cohesive toposes, there exists a notion of contractability, and toposes with a subobject classifier $\Omega$ that is contractible are of special interest (see here).

It occured to me that one could view probability theory as a theory of (convex) interpolation of truth, which replaces boolean truth $\Omega = \{0,1\}$ by 'continuous truth' $\Omega = [0,1]$, so I wonder, is there an example of a topos with a subobject classifier given (in a suitable sense) by the real unit interval?

Best Answer

Here's an example which, I'm afraid, is not very interesting (and may not match your notion of “suitable sense”): let $X$ be the topological space $\mathopen]0,1\mathclose[$ (the open interval) with the (not at all separated) topology given by the $U_x := \mathopen]0,x\mathclose[$ for $x \in [0,1]$ (with $U_0 = \varnothing$, obviously). Since $U_x \cap U_y = U_{\min(x,y)}$ and $$ \bigcup_{i\in I} U_{x_i} = U_{\sup\{x_i : i\in I\}} $$ this is indeed a topology. Of course, $\mathcal{O}(X) := \{U_x : x\in[0,1]\}$ can be identified with $[0,1]$ as an ordered set (indeed, frame). The topos of sheaves on $(X, \mathcal{O}(X))$ then has this as subobject classifier.

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