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Jun 2, 2022 at 9:15 comment added Taras Banakh @JochenWengenroth Of course, you are right about $m$. The sequentially closed subset of $M_1\times M_2$ which is not closed is the ``diagonal'' $\{(z,z): z\in M_1\setminus\{0\}\}$ of $M_1\times M_2$.
Jun 2, 2022 at 9:10 history edited Taras Banakh CC BY-SA 4.0
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Jun 2, 2022 at 9:04 comment added Jochen Wengenroth The elements of $M_1$ are probably $1/n+i/mn$ with $n\in\mathbb N$ and $m\in\mathbb N$? What is a sequentially closed subset of $M_1\times M_2$ which is not closed?
Jun 2, 2022 at 3:45 comment added Taras Banakh @rmcerafl Yes, I mean that the product topology on $M_1\times M_2$ can be non-sequential. If the product topology is sequential, then I do not know the answer.
Jun 1, 2022 at 21:57 comment added fsp-b (Also (excuses for my naivety but I'm far from being a topologist): If you say that ''the product $M_1\times M_2$ is not sequential'' then you do mean that the product topology on $M_1\times M_2$ is not sequential, right?)
Jun 1, 2022 at 21:48 comment added fsp-b Thank you, @Taras! Do you know if the question can be answered positively if the product $M_1\times M_2$ is assumed to be sequential?
Jun 1, 2022 at 21:41 vote accept fsp-b
Jun 1, 2022 at 20:41 history answered Taras Banakh CC BY-SA 4.0