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Nov 22, 2017 at 12:44 vote accept Dominic van der Zypen
Nov 22, 2017 at 12:44 comment added Dominic van der Zypen That's right, thanks for the picture!
Nov 22, 2017 at 12:43 comment added Todd Trimble My linked answer says that subbasis elements like $\pi^{-1}(U)$ are not open in the interval topology, because $\pi_1^{-1}(U)$ is too small to contain any union of a northwest quadrant and a southeast quadrant (and any basic in the interval topology contains such a union). In brief, your displayed formula for $U$ is incorrect. Try drawing a picture of a $P \setminus (\uparrow (x, y))$, which looks like a complement of a northeast quadrant (i.e., looks like a "thick L-shape"), and you should see this easily.
Nov 22, 2017 at 12:29 comment added Dominic van der Zypen But why is every subbasic set of the product topology $\tau_p$ open in the interval topology (see my (non-)answer, starting from "Conversely")
Nov 22, 2017 at 11:03 history answered Todd Trimble CC BY-SA 3.0