I have a assigned worksheet that claims so.
"This is irreflexive, as $x – x = 0$, which is never $> 1$.
This is neither symmetric nor antisymmetric, example: $x=5, y=1$ works, $y=5, x=1$ doesn’t (would imply antisymmetric), but $x=2, y=1$ doesn’t work and neither does $x=1, y=2$, so can’t be antisymmetric.
This is transitive. If the difference between $x$ and $y$ is greater than $1$ and between $y$ and $z$ is greater than $1$, then the difference between $x$ and $z$ must be greater than $2$."
I can appreciate that the relation is not symmetrical but am having confusions about the anti-symmetry.
The way I understand anti-symmetry in relations is that if $(x,y)$ values exist such that $xRy$ and $yRx$, then $x=y$. Now, since there are no values $(x,y)$ that hold the relation, we cannot dismiss this anti-symmetry. I have stumbled upon similar cases in discrete mathematics where if you cannot dismiss a hypothesis, it is accepted as true. Wouldn't it be true here too?
Why does $x=2, y=1$ doesn’t work and neither does $x=1, y=2$ dismiss anti-symmetry? Which condition of anti-symmetry is this breaking?
It would be helpful if you could also state its transitivity and reflexiveness.
Slide from my lecture regarding anti-symmetry.
Best Answer
Let's recall the definitions. A relation $\sim$ is $\textit{reflexive}$ if $x\sim x$, $\textit{transitive}$ if $x\sim y$ and $y\sim z$ imply $x\sim z$, $\textit{symmetric}$ if $x\sim y$ implies $y\sim x$, and, $\textit{antisymmetric}$ if $x\sim y$ implies $¬(y\sim x)$ instead.
The relation used above is "$x\sim y$ if $x-y>1$". Let's check the properties.
So this relation is transitive and antisymmetric, but not reflexive. Antisymmetry excludes symmetry.