2

Can someone please help?

I am trying to answer the following:

Consider the relation $T$ on $\mathbb{Z}$ given by $$xTy \Longleftrightarrow x + 1 \le y;$$ Is $xTy$ asymmetric?

$xTy \Rightarrow x\neg Ty$

$xTy \Leftrightarrow x+1 \le y$

$x\neg Ty \Leftrightarrow y+1>y $

We assume that $x \lt y$ is true, then $y+1 \gt x$ will always be true.

Then $xTy$ is asymmetric.

Is this a good enough or even valid proof?

Thanks

Arturo Magidin
  • 398,050
Gunnar
  • 133
  • 1
    Can you explain what $,_xNOT_y$ means? – Srivatsan Sep 21 '11 at 17:54
  • @SrivatsanNarayanan I think he means not related – Ragnar123 Sep 21 '11 at 17:55
  • 3
    You can use $\not T$ or $\lnot T$ if that is what you want. Right click and choose Show Source to see what I wrote. – Ross Millikan Sep 21 '11 at 17:58
  • Yes, it means Not related, but I did not figure out how to make a not T. - I have updated it now, so it is $¬T$ – Gunnar Sep 21 '11 at 17:58
  • Regarding "$_xT_y \Rightarrow _xNOT_y$". Assuming that $x$ is related to $y$, how can you show that $x$ is not related to $y$ as well? Are you trying to show $y \not T x$? – Srivatsan Sep 21 '11 at 18:01
  • Being not proficient in math, I try to prove that the relation $_xT_y$ is asymmetrical. Help me out :-) – Gunnar Sep 21 '11 at 18:04
  • The notation $_xT_y$ is really strange. In any case there seem to be many typos in the question. Should it not be $_xT_y \Rightarrow _y¬T_x$? And $_y¬T_x \Leftrightarrow y+1 > x$? – TMM Sep 21 '11 at 18:07
  • Thijs, yes you are right. I will update. - Now it is updated. So are we any closer? :-) – Gunnar Sep 21 '11 at 18:19
  • 2
    What kind of objects are $x$ and $y$? This is a relation among what? Integers? Real numbers? $x+1\leq y$ is equivalent to $x\lt y$ for integers, but not for rational or real numbers. $x+1\not\leq y$ is equivalent to $y+1\leq x$ for integers, but not for rational or real numbers. You need to specify not just the "rule" for the relation, but what it is a relation on. – Arturo Magidin Sep 21 '11 at 18:21
  • 1
    Note: "asymmetric" and "symmetric" are adjectives that apply to the relation $T$; but "$xTy$" is a statement that says that $x$ is $T$-related to $y$. Technically, "$xTy$" is neither symmetric nor asymmetric, because it is not a relation, it's a statement that says that a particular ordered pair lies in the set $T$. – Arturo Magidin Sep 21 '11 at 18:26
  • Sorry for missing this out in the first post, but i got this "Consider the relation T on Z given by" – Gunnar Sep 21 '11 at 18:30
  • @Sylvester: $x\neg Ty \Leftrightarrow y+1>y$ still makes no sense. – TMM Sep 21 '11 at 19:05

1 Answers1

3

A relation $T$ is asymmetrical if and only if whenever $xTy$ holds, $yTx$ does not hold. That is, if and only if: $$xTy\Longrightarrow y(\neg T) x.$$

So what you write makes no sense in terms of proving this. You write that you want to prove that if $xTy$ holds, then $x(\neg T)y$ is true; but this is not what you need to prove.

Also, your definition is that $xTy$ holds if and only if $x+1\leq y$; then you claim that $x(\neg T)y$ is equivalent to $y+1\leq x$; while that may be (see below) a true deduction, it's better to write what the condition means directly: if $xTy$ is equivalent to $x+1\leq y$, then $x(\neg T)y$ is equivalent to $\neg(x+1\leq y)$, which is equivalent to $x+1\not\leq y$.

Now, you don't specify what kind of numbers you are dealing with. For real numbers, $x+1\not\leq y$ is equivalent to $x+1\gt y$; but this is not the same as $y+1\leq x$ if you are dealing with real numbers, or with rational numbers. It's true for integers, but since you don't specify which kinds of $x$s you are dealing with, it could also be incorrect.

I think there are too many errors and unstated assumptions to make what you write a correct proof.

So, is this relation asymmetrical? For real numbers:

We want to prove that if $xTy$, then $y(\neg T)x$. So, assume that $xTy$. That means that $x+1\leq y$. We need to show that $y(\neg T)x$, meaning we need to show that $y+1\not\leq x$.

Indeed, we know that $x\lt x+1\leq y$; therefore, $x\lt y\lt y+1$. By transitivity, if $y+1\leq x$, then we would have $x\lt x$, which is impossible. Therefore, $y+1\not\leq x$. Therefore, $y(\neg T)x$.

Since the relation is asymmetrical for real numbers, it is also asymmetrical for rationals, integers, or natural numbers.

Arturo Magidin
  • 398,050