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We know that every Partial Order Set has to satisfy three conditions :

1- Reflexive

2- Anti-Symmetric

3- Transitive

For example : $$S= \left(\left\{1 \right\},\left\{2 \right\},\left\{3 \right\},\left\{1,2 \right\} \right)$$

And the relation is $\subseteq$ ,My question is why this Set S is a Partial Order Set Althought I Cannot prove that is Transitive which is defined as:

$$R=\{(x,y)\} ; xRy ~\text{and}~ yRz \implies xRz$$ I could not find like : if $\left\{1 \right\} \subseteq \left\{1,2 \right\}\text{ } and \text{ } \left\{1,2 \right\} \subseteq \text{ }\text{ } ?? \text{ }then\text{ } ??$

in another way how can we apply the Transitive test for this set S?

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  • $\begingroup$ Recall the definition of a transitive relation: A relation $R$ on a set $A$ is transitive if whenever $(a,b)\in R$ and $(b,c)\in R \implies (a,c)\in R$ for $a,b,c \in A$. $\endgroup$
    – user371838
    Commented Dec 8, 2017 at 6:54

1 Answer 1

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Note that statement of the form of "if $A$ then $B$" is true if $A$ is false.

Well, actually we have $\color{blue}{\{1 \}} \subseteq \color{green}{\{ 1,2\}}$ and $\color{green}{\{1,2\}} \subseteq \color{purple}{\{ 1, 2\}}$, then $\color{blue}{\{1\}} \subseteq \color{purple}{\{1,2\}}.$

Notice that for the subset operation, we always have

$$x \subseteq y \text{ and }y \subseteq z \text{ ,then }x \subseteq z$$

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  • $\begingroup$ like that .. i understand the first line but do you mean that y may equel to z? $\endgroup$
    – Yasser Milato
    Commented Dec 8, 2017 at 6:57
  • $\begingroup$ There is no restriction that $x$, $y$ and $z$ have to be distinct right? we are suppose to check all possibilities. though the case for identical elements are trivial. Most important thing should be the last line to show transitivity. $\endgroup$
    – Siong Thye Goh
    Commented Dec 8, 2017 at 7:01
  • $\begingroup$ Perfect then i can understand that A relation ⊆ on a set A: is always Transitive .. right ? Where A is a Set of at least One Element . $\endgroup$
    – Yasser Milato
    Commented Dec 8, 2017 at 7:07
  • $\begingroup$ yes, subset is a transitive relation. $\endgroup$
    – Siong Thye Goh
    Commented Dec 8, 2017 at 7:10
  • $\begingroup$ thanks a lot bro.. in the definition of a transitive relation: A relation R on a set A is transitive if whenever (a,b)∈R(a,b)∈R and (b,c)∈R⟹(a,c)∈R(b,c)∈R⟹(a,c)∈R for a,b,c∈A ... could a,b and c be equel ? @rohan $\endgroup$
    – Yasser Milato
    Commented Dec 8, 2017 at 9:43

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