This question was inspired by this "beautiful mind" question, where some one asks about the mathematical contents of some movie. I do not see how this other question is less opinion-based than the one I am posing. This post also did not motivate anyone to censor it...

I am giving a summer course on topology starting on January 6th. I am thinking about showing the Hitler Learns Topology meme on our first class, and maybe, showing again on the last class, or after the chapter on connectedness.

But I have some doubts on the mathematical contents of the video, and I would be really thankful if someone could clarify those to me.

OF COURSE, since the problem is NOT fully shown, I just hope someone can recognize the theory they are talking about and explain it to me.

  1. What is the "null set" they are talking about?

  2. Is this "null set" the topological space they are talking about? Or is it just the "clopen" set in question?

  3. I understood that the clopen set in question is "the zero of the points in the null set". What is this set? And why is it clopen?

  4. What is the "close points" Hitler refers to?

Edit: I have edited the question so as to make it absolutely mathematical.

  • $\begingroup$ It is trash. Do not ever show it. Toss it away. A null set is a set with zero measure which is off topic for simple point set topology. $\endgroup$ Dec 31, 2019 at 12:18
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    $\begingroup$ @WilliamElliot I think in this context "null set" just means "empty set". $\endgroup$
    – littleO
    Dec 31, 2019 at 12:32
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    $\begingroup$ I didn't understand the first 15 seconds of the video, but I think it's a pretty entertaining video. It might spice things up a bit and also provide some sympathy for the students who have to learn so many new strange-sounding definitions. $\endgroup$
    – littleO
    Dec 31, 2019 at 12:34
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    $\begingroup$ Do not show this video. Whatever its mathematical contents may be, it will grossly trivialise Hitler, genocide and crimes against humanity. The film-scene is from the movie "Downfall" (2005) and is about a meeting of Hitler and his General Staff in the last days of the Second World War just before the Soviet army took Berlin. In my opinion it is a shame to play down Hitler and Nazi Germany by fake subtitles. The movie itself is worth seeing: It shows the monster Adolf Hitler and his entourage. $\endgroup$
    – Paul Frost
    Dec 31, 2019 at 13:51
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    $\begingroup$ I find this to be an odd platform for a borderline antisemitic rant. Please don't do that in the future. If you have opinions about people based on their religion/ethnicity/nationality, we expect that you keep them to yourself (and this is a good tip not just on this platform). $\endgroup$
    – Asaf Karagila
    Jan 3, 2020 at 7:36

2 Answers 2


The null set is just $\emptyset$, the empty set. This is open by definition (and closed too as its complement ($X$, the whole space) is also open (by definition). Normally a null set is reserved for measure theory, and a zero-set is a notion from later in topology: a set of the form $f^{-1}[\{0\}]$ for some real-valued continuous function on $X$.

The null set is not the space they're talking about, AFAIKT.

A clopen set is indeed a set that is both open and closed, it's a usual term (we even use it in Dutch normally, though an older professor of mine preferred "opgesloten" as a Dutch alternative ("gesloten"=closed, "open" = open), but that already means "locked up", so sounds confusing to me personally).

I think "close point" is just a mistranslation of "adherent point" ($x$ is an adherent point of $A$ whenever all neighbourhoods of $x$ intersect $A$) and a set is closed if it equals the set of all its adherent points. "adherent" etymologically means something like "glued to", "clinging to" (from Latin adhaerere.)

The empty set (in that last definitional view) is closed because it has no adherent points, so it indeed contains all its adherent points as a vacuous truth.

I think it's cleaner to define closed sets directly after the introduction of open sets as their complements. But then having notions of interior and adherence and boundary points can be handy as well, and all are easily defined in terms of open sets.

  1. It's a synonym for the empty set, $\emptyset$. (Not recommended, since it means something else in measure theory.)
  2. No (see previous answer).
  3. It says “all the zero of the points in the null set have a neighborhood...”, which I think is supposed to mean “all the points in the empty set (and there are no such points, their number is zero, there is zero of them) have a neighborhood...”. And this is a vacously true statement, which can be confusing for beginners; if there are no points in the set $S$, then any statement of the form “for all $x \in S$ this or that holds” counts as true, since there are no points in $S$ that could falsify it. The clopen set that they are talking about is the empty set.
  4. No idea. Maybe a direct translation of a topological term from some other language? (Boundary point, perhaps?)

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