Why is the boundary of the rationals (Q) equal to R?

[filter54321]

I was reading a website that said the boundary of a set's boundary is equal to the first boundary. Visually, this makes sense for subsets of R1 and R2 because the first boundary will not have an interior (no ball about the points will fall into the boundary).

However, the reading went on to say that the boundary of the rationals Q is R. This seems wrong to me so I am questioning the entire site.

Wouldn't the boundary of Q be Q? A ball of positive radius about any point in Q would contain both points from Q as well as irrationals from P. ddQ = dQ = Q (where d is boundary operator). The theorem at top does appear to hold but the example is messed up...no?

 

[quasar987]

If A is a subset of R^n, then a boundary point of A is, by definition, a point x of R^n such that every open ball about x contains both points of A and of R^n\A.

So for instance, in the case of A=Q, yes, every point of Q is a boundary point, but also every point of R\Q because every irrational admits rationals arbitrarily close to it. So in the end, dQ=R.

 

[Tinyboss]

The rationals in the reals are good for all kinds of examples and counterexamples.

Here's a cool one: there is a proper open subset $$U\subset\mathbb{R}$$ with $$\mathbb{Q}\subset U$$. That is, U is open and contains all of Q, but not all of R.

 

[quasar987]

Isn't it obvious? Take U=R minus any irrational point.

 

[blue_raver22]

I would like to clarify that the boundary of a set is defined as the set of points that are both inside and outside of the given set. In other words, it is the set of points where the set "ends" and the outside space "begins".

In the case of the set of rational numbers (Q), the boundary is indeed equal to Q. This is because every rational number has both rational and irrational numbers on either side of it, making it a point on the boundary. Therefore, a ball of positive radius around any point in Q will contain both rational and irrational numbers, and thus the boundary is equal to Q.

The statement on the website that the boundary of Q is R is incorrect. The set of real numbers (R) includes both rational and irrational numbers, but it is not the same as the boundary of Q. It is important to note that the boundary of a set is not necessarily equal to the set itself.

In conclusion, the boundary of the rationals (Q) is equal to Q, not R. As a scientist, it is important to critically evaluate information and question sources that may not be accurate.