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That was what I first thought, too, but it is wrong. We have a description of any open set, not only basis: read the qualifiers! Open is, what contains an open (half-open) interval of all of its elements. This already covers infinite unions.

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##\emptyset \subseteq \tau,## really? And ##X:=\cup \tau## is more artificial than ##X\in \tau## is. It does not work without that setting, since ##\left(\left\{a,b\right\};\left\{\emptyset,\{a\},\{a,b\}\right\}\right)## is a topological space and ##\left(\left\{a,b\right\};\left\{\{a\},\{a,b\}\right\}\right)## or ##\left(\left\{a,b\right\};\left\{\emptyset,\{a\}\right\}\right)## are not.PeroK said:

This is not minimal, these are hidden assumptions, shell game tricks.

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The full set will necessarily result from the union of all sets in the topology. The empty set then can be derived through DeMorgan or similar.PeroK said:My point, for what it's worth, is that you can drop the first axiom about the empty set and full set being in the topology - as long as you recognise the empty intersection and empty union in the other axioms.

It doesn't practically change what you have to test, since checking the empty intersection and empty union are special cases that amount to checking that the empty set and full set are in the topology! But, you can save yourself an axiom if you want.

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This is a tautology. "The full sets results from the topology." You need toWWGD said:The full set will necessarily result from the union of all sets in the topology. The empty set then can be derived through DeMorgan or similar.

Every other view on it is shell game mathematics.

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You don't need to define it that way. Once you're provided with all sets in the topology, the definition warrants that their union, which will include the entire space, is in the topology.fresh_42 said:This is a tautology. "The full sets results from the topology." You need todefine##X:=\cup \tau.## Otherwise, you simplydo notget the full set. There isnoway to get ##\{a,b\}## from ##\tau\stackrel{?}{=}\{\emptyset,\{a\}\}## without putting it into ##\tau ##first.

Every other view on it is shell game mathematics.

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No. You use a tautology to achieve that result. What is wrong with my example?WWGD said:You don't need to define it that way. Once you're provided with all sets in the topology, the definition warrants that their union, which will include the entire space, is in the topology.

Space ##X:=\{a,b\}.## Open sets ##U_1=\emptyset## and ##U_2=\{a\}.##

Now construct a topology from that without

Shell game, but neither logic nor mathematics.

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Every element in the space has to be " covered " ( included in) some open set. Otherwise missing elements are somehow extraneous. If you can't define neighborhoods of some points, as in b here, those points are not part of your topological space. But this all ultimately becomes more of an issue of semantics, maybe philosophy. Not sure how productive these exchanges can be.fresh_42 said:No. You use a tautology to achieve that result. What is wrong with my example?

Space ##X:=\{a,b\}.## Open sets ##U_1=\emptyset## and ##U_2=\{a\}.##

Now construct a topology from that withoutdefining##U_3=:X.##

Shell game, but neither logic nor mathematics.

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... and the union of no sets produces the empty set.

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Shell game. My speech. "Put them into have them in." A ridiculous argument in my mind. Fact is: you cannot generate ##\{a,b\}## without defining that ##b## has to be part of some open set? And how is that any different from ##\{a,b\}\in \tau?##WWGD said:Every element in the space has to be " covered " ( included in) some open set. Otherwise missing elements are somehow extraneous. If you can't define neighborhoods of some points, as in b here, those points are not part of your topological space. But this all ultimately becomes more of an issue of semantics, maybe philosophy. Not sure how productive these exchanges can be.

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... although the above link highlights some issues with the former on set-theoretic grounds.

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Its more ridiculous to define a topology that does not allow you to include elements to which it applies. How about defining your topology on the Reals themselves with just the empty set and ##\{a\}##? Then you can't really speak of anything but the element a itself. Topology on X is a scheme to say something about X and its elements. But if you don't include an element in an open set, you can't say anything about it, which is just absurd. I guess to me you start with a set and define a topology in it, not the other way around.fresh_42 said:Shell game. My speech. "Put them into have them in." A ridiculous argument. Fact is: you cannot generate ##\{a,b\}## without defining that ##b## has to be part of some open set? And how is that any different from ##\{a,b\}\in \tau?##

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You do notWWGD said:Its more ridiculous to define a topology that does not allow you to include elements to which it applies.

I am absolutely sure that we wouldn't have ##\emptyset\, , \,X \in \tau## as an axiom if it wasn't necessary. Pawel Samuilowitsch Urysohn, Ascher Zaritsky, Felix Hausdorff, Andrej Nikolaevic Tichonov, and Hans Julius Zassenhaus would have long dropped it!

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You may find it a ridiculous topology, but ##\{\emptyset, \{a\}, X\}##WWGD said:Its more ridiculous to define a topology that does not allow you to include elements to which it applies. How about defining your topology on the Reals themselves with just the empty set and ##\{a\}##? Then you can't really speak of anything but the element a itself. Topology on X is a scheme to say something about X and its elements. But if you don't include an element in an open set, you can't say anything about it, which is just absurd. I guess to me you start with a set and define a topology in it, not the other way around.

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Well, it's clearer with a separate axiom than relying on an ugly and somewhat artificial interpretation of unions and intersections. And, as pointed out in an earlier post, it doesn't save any practical effort in checking that a topology satisfies the axioms.fresh_42 said:You do notallowit, you demand it! And that makes it equivalent to ##X\in \tau## only under the shell.

I am absolutely sure that we wouldn't have ##\emptyset\, , \,X \in \tau## as an axiom if it wasn't necessary. Pawel Samuilowitsch Urysohn, Ascher Zaritsky, Felix Hausdorff, Andrej Nikolaevic Tichonov, and Hans Julius Zassenhaus would have long dropped it!

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Edit: I never said a particular element should be contained in a particular set. Both the discrete and indiscrete topologies are used for little but counterexamples . In the Discrete topllogy on X, every map out of X is continuous. In the Indiscrete, elements in X can't be separated or distinguished. Not much useful comes out of either.Orodruin said:You may find it a ridiculous topology, but ##\{\emptyset, \{a\}, X\}##isa perfectly good topology while ##\{\{a\}\}## is not. There is no requirement that any open setexcept the full set contains any particular element. Since the full set is included by axiom, that each point has a neighbourhood becomes trivially true and uninteresting. There is no requirement for a topology that the full set should result from the union of any set of open sets that does not include the full set. In fact, any set ##X## can be equipped with the trivial topology ##\{\emptyset,X\}## where it is obvious that you cannot obtain ##X## from any union of open sets that does not already include ##X##. This is a matter of fact, not opinon, although you are free to dislike the fact.

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I know, that was my example. And ##\{\{a\}\}## isn't a topology on ##\{a,b\}## even if we included ##\{\emptyset,\{a\}\}.##Orodruin said:You may find it a ridiculous topology, but ##\{\emptyset, \{a\}, X\}##isa perfectly good topology while ##\{\{a\}\}## is not.

I find the request ridiculous that every element has to be part of some open set, neither the topology nor the persons who make that claim. I am saying that this is of equal quality as ##\{a,b\}\in \tau## is, and only disguises this fact. It complicates things by hiding the entire axiom behind

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You did implicitly. Without such a statement you will have no chance to prove that ##\tau:=\{\emptyset\, , \,\{a\}\}## is no topology on ##X:=\{a,b\}.## As I said before, there is no way to create ##X## as a union of sets from ##\tau.## You have to make an assumption that is basically equivalent to ##X\in \tau,## only hidden in the right shell of the shell gamer.WWGD said:I never said a particular element should be contained in a particular set.

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I wish you didn't have to hurl accusations like shell gamer. I guess I should have stood by my decision to stay out, while standing by what I've said.fresh_42 said:You did implicitly. Without such a statement you will have no chance to prove that ##\tau:=\{\emptyset\, , \,\{a\}\}## is no topology on ##X:=\{a,b\}.## As I said before, there is no way to create ##X## as a union of sets from ##\tau.## You have to make an assumption that is basically equivalent to ##X\in \tau,## only hidden in the right shell of the shell gamer.

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That is my impression. The axiom is hidden beneath the notation such that it gets invisible, only to say it is there at the end of the argument. I had this association from the stackexchange post @PeroK has linked to. They hid it under ##X =\cup \tau.## This is only another way to say that every element has to be in an open set, which is another way to say ##X\in \tau.##WWGD said:I wish you didn't have to hurl accusations like shell gamer. I guess I should have stood by my decision to stay out, while standing by what I've said.

The shell game association came from

which looked to me like hiding ##X\in \tau## in the notation ##\tau## is a topological space, which seems to me a ταυτολογία.

But to make it clear: By no means I did ever suggest that you are a shell gamer or even have any similarities with one. The minimal system of axioms actually works for the topologies as defined in post #1. They do not work for the example I gave. So either we drop the requirement that a definition has to cover all cases, or we will have to use tricks like the one on stackexchange.

I mean, read it again dear @WWGD!

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You kind of did, at least you make it seem like it.WWGD said:Edit: I never said a particular element should be contained in a particular set.

This

is not a topology on ##\mathbb R## because ##\mathbb R## is not in the topology so what follows simply does not apply. However, if you do include ##\mathbb R## then it is a topology and the statement that every ##x \in \mathbb R## must be in an open set is trivial because ##x \in \mathbb R##, which is open.WWGD said:How about defining your topology on the Reals themselves with just the empty set and {a}?

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A standard argument/technique is to show that every element on the left hand is contained in the set on the right hand.patric44 said:Thanks for your response. But this still not clear for me, I wish there is a rigorous approach for your statement. Suppose that ##\delta=1##, we have

$$[x,x+1)\subset(x-1,x+1).$$

I'm not sure that this is still true.

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That was the whole point. A topology defined on a set X must be able to talk about anything in X. Here {a} stood for the set in which the topology is defined, where {a} is any Real number.Orodruin said:You kind of did, at least you make it seem like it.

This

is not a topology on ##\mathbb R## because ##\mathbb R## is not in the topology so what follows simply does not apply. However, if you do include ##\mathbb R## then it is a topology and the statement that every ##x \in \mathbb R## must be in an open set is trivial because ##x \in \mathbb R##, which is open.

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