No Arabic abstract
In The factorization of the Giry monad (arXiv:1707.00488v2) the author considers two $sigma$-algebras on convex spaces of functions to the unit interval. One of them is generated by the Boolean subobjects and the other is the $sigma$-algebra induced by the evaluation maps. The author asserts that, under the assumptions given in the paper, the two $sigma$-algebras coincide. We give examples contradicting this statement.
For a function $fcolon [0,1]tomathbb R$, we consider the set $E(f)$ of points at which $f$ cuts the real axis. Given $fcolon [0,1]tomathbb R$ and a Cantor set $Dsubset [0,1]$ with ${0,1}subset D$, we obtain conditions equivalent to the conjunction $fin C[0,1]$ (or $fin C^infty [0,1]$) and $Dsubset E(f)$. This generalizes some ideas of Zabeti. We observe that, if $f$ is continuous, then $E(f)$ is a closed nowhere dense subset of $f^{-1}[{ 0}]$ where each $xin {0,1}cap E(f)$ is an accumulation point of $E(f)$. Our main result states that, for a closed nowhere dense set $Fsubset [0,1]$ with each $xin {0,1}cap E(f)$ being an accumulation point of $F$, there exists $fin C^infty [0,1]$ such that $F=E(f)$.
We make some beginning observations about the category $mathbb{E}mathrm{q}$ of equivalence relations on the set of natural numbers, where a morphism between two equivalence relations $R,S$ is a mapping from the set of $R$-equivalence classes to that of $S$-equivalence classes, which is induced by a computable function. We also consider some full subcategories of $mathbb{E}mathrm{q}$, such as the category $mathbb{E}mathrm{q}(Sigma^0_1)$ of computably enumerable equivalence relations (called ceers), the category $mathbb{E}mathrm{q}(Pi^0_1)$ of co-computably enumerable equivalence relations, and the category $mathbb{E}mathrm{q}(mathrm{Dark}^*)$ whose objects are the so-called dark ceers plus the ceers with finitely many equivalence classes. Although in all these categories the monomorphisms coincide with the injective morphisms, we show that in $mathbb{E}mathrm{q}(Sigma^0_1)$ the epimorphisms coincide with the onto morphisms, but in $mathbb{E}mathrm{q}(Pi^0_1)$ there are epimorphisms that are not onto. Moreover, $mathbb{E}mathrm{q}$, $mathbb{E}mathrm{q}(Sigma^0_1)$, and $mathbb{E}mathrm{q}(mathrm{Dark}^*)$ are closed under finite products, binary coproducts, and coequalizers, but we give an example of two morphisms in $mathbb{E}mathrm{q}(Pi^0_1)$ whose coequalizer in $mathbb{E}mathrm{q}$ is not an object of $mathbb{E}mathrm{q}(Pi^0_1)$.
For a category $mathbb{C}$, a small category $mathbb{I}$, and a pre-cover relation $sqsubset$ on $mathbb C$ we prove, under certain completeness assumptions on $mathbb C$, that a morphism $g: Bto C$ in the functor category $mathbb {C}^{mathbb I}$ admits an image with respect to the pre-cover relation on $mathbb C^{mathbb I}$ induced by $sqsubset$ as soon as each component of $g$ admits an image with respect to $sqsubset$. We then apply this to show that if a pointed category $mathbb{C}$ is: (i) algebraically cartesian closed; (ii) exact protomodular and action accessible; or (iii) admits normalizers, then the same is true of each functor category $mathbb{C}^{mathbb I}$ with $mathbb{I}$ finite. In addition, our results give explicit constructions of images in functor categories using limits and images in the underlying category. In particular, they can be used to give explicit constructions of both centralizers and normalizers in functor categories using limits and centralizers or normalizers (respectively) in the underlying category.
We show that doubly degenerate Penon tricategories give symmetric rather than braided monoidal categories. We prove that Penon tricategories cannot give all tricategories, but we show that a slightly modified version of the definition rectifies the situation. We give the modified definition, using non-reflexive rather than reflexive globular sets, and show that the problem with doubly degenerate tricategories does not arise.
Frobenius monoidal functors preserve duals. We show that conversely, (co)monoidal functors between autonomous categories which preserve duals are Frobenius monoidal. We apply this result to linearly distributive functors between autonomous categories.