# Density of Q in R

• Nov 15th 2013, 02:13 PM
davidciprut
Density of Q in R
Can someone prove (and explain step by step the proof) the density of Rationals in Real numbers?

By the way, in my notebook they gave the density of rationals in real numbers as definition, my second question is, definitions are things that we define and we can't prove them so why is there a proof? Maybe it's not a definition but a statement ? Or do definitions have proofs too? Thank you in advance.
• Nov 15th 2013, 02:58 PM
Plato
Re: Density of Q in R
Quote:

Originally Posted by davidciprut
Can someone prove (and explain step by step the proof) the density of Rationals in Real numbers?

I will not do that. But I will give you an outline.
1) If $r\in\mathbb{R}$ then $(\exist j\in\mathbb{Z})[j\le r. (Think floor function).

2) If $y-x>1$ then $(\exists K\in\mathbb{Z})[x < K< y]$,
Use 1) note that $\left\lfloor x \right\rfloor \le x < \left\lfloor {x } \right\rfloor +1$ or $\left\lfloor x \right\rfloor +1 \le x+1 < y$ So $x < \left\lfloor {x } \right\rfloor+1.

3) If $0 then $\left( {\exists J \in\mathbb{ Z}^+} \right)\left[ {J > \frac{1}{{s - t}}} \right]$ of course that means $Js-Jt>1$. Use #2.

4) You get $Js>K>Jt$. Divide by $J$.
• Nov 15th 2013, 03:02 PM
HallsofIvy
Re: Density of Q in R
I am puzzled as to what you are trying to say.

Quote:

Can someone prove (and explain step by step the proof) the density of Rationals in Real numbers?
Prove what about density of the Rationals in the Real numbers? "The density of the Rationals in the Real numbers" is a noun phrase. We prove statements, not phrases.

Quote:

By the way, in my notebook they gave the density of rationals in real numbers as definition, my second question is, definitions are things that we define and we can't prove them so why is there a proof? Maybe it's not a definition but a statement ? Or do definitions have proofs too? Thank you in advance.
We define nouns, we prove statements. We might include some properties in the definition of a noun, rather than proving them. But, exactly what was this definition you are referring to? I know a definition of one set being "dense" in a superset, given some topology. Give such a definition, one would then have to prove that this definition applied to the rational numbers as a subset of the real numbers, given the usual topology.

For example, we can say that any set, X, is "dense in the real numbers" if and only if, given any real number, y, and real number $\lambda> 0[/tex], there exist a number, x, in X, such that [itex]|x- y|< \lambda$. We could then show, using that definition, that Q is dense in R.