Quique Ruiz
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 Apr9 revised Is the cartesian product of homeomorphisms again a homeomorphism? added 2 characters in body Apr9 answered Is the cartesian product of homeomorphisms again a homeomorphism? Apr8 comment Is the cartesian product of homeomorphisms again a homeomorphism? What about a categorical argument? It's very simple. Mar30 awarded Commentator Mar30 comment why natural transformatoins are also called “morphisms” of functors? Maybe what you're looking for is in Exercise I.4.5 of Categories for the working mathematician. Mar14 awarded Teacher Mar13 accepted “for almost all” symbol Mar13 asked “for almost all” symbol Mar7 awarded Critic Mar7 comment Examples of colimits in a category of categories Your question reminds me this paper. They give some examples of coequalizers in $\mathbf{Cat}$. Mar7 answered Problem book on general topology Mar6 comment Question of regular open $U(A)=\cup\{U \text{ open}\mid A\subseteq U\}$? Mar6 comment Properties of Closure This is not true. Consider $\mathbf{R}$ with the usual topology. Let $A:=[0,1)$. Then $(\bar{A})^c=(-\infty,0)\cup(1,\infty)$, but $\bar{A^c}=(-\infty,0]\cup[1,\infty)$. Feb4 comment How to show the standard $n$-simplex is homeomorphic to the $n$-ball There is a generalization of this in Bredon's Topoloy and geometry, Proposition 16.4, page 56. The proposition says: ''A compact convex body $C$ in $\mathbf{R}^n$ with nonempty interior is homeomorphic to the closed $n$-ball''. Jan19 accepted Boundary, unions and intersections Jan6 accepted Subbases and half-planes Jan6 asked Boundary, unions and intersections Dec20 asked Subbases and half-planes Nov27 accepted Idempotents in $\mathbf{CRing}$ Oct29 asked Idempotents in $\mathbf{CRing}$