# A Proof of Posets, how to do it?

I am trying to do a exercise from my book, but I don't understand how to solve it (many statements!)...

Here is the exercise:

Let $P$ and $Q$ be ordered sets. Prove that $(a_1, b_1)\prec (a_2,b_2)$ in $P\times Q$ if and only if ($a_1 = a_2$ and $b_1\prec b_2$) or ($a_1\prec a_2$ and $b_1 = b_2$).

$x\prec y$ means $x$ covers $y$: $x \le y$ and there does not exist $z$ such that $x\le z\le y$.

And $(a_1,b_1)\prec (a_2,b_2) \iff a_1\le a_2\text{ and }b_1 \le b_2$ with covering in each case.

The problem is that I don't know how to prove because of so many statements from left to right direction... Can anyone help me??

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HINT: It’s very straightforward to prove that if $a_1=a_2$ and $b_1\prec b_2$, or if $a_1\prec a_2$ and $b_1=b_2$, then $\langle a_1,b_1\rangle\prec\langle a_2,b_2\rangle$; I’ll leave that to you. Now suppose that $\langle a_1,b_1\rangle\prec\langle a_2,b_2\rangle$; you need to show that either $a_1=a_2$ and $b_1\prec b_2$, or $a_1\prec a_2$ and $b_1=b_2$.

Certainly $\langle a_1,b_1\rangle\le\langle a_2,b_2\rangle$, so $a_1\le a_2$ and $b_1\le b_2$.

• Suppose that $a_1=a_2$, and $b_1\le b_2$ but $b_1\nprec b_2$; then there is a $b\in B$ such that $b_1<b<b_2$, and clearly $\langle a_1,b_1\rangle<\langle a_1,b\rangle<\langle a_2,b_2\rangle$, contradicting the assumption that $\langle a_1,b_1\rangle\prec\langle a_2,b_2\rangle$.

• Suppose that $b_1=b_2$, and $a_1\le a_2$ but $a_1\nprec b_1$; use a similar argument to get a contradiction.

• Suppose that $a_1<a_2$ and $b_1<b_2$; get a contradiction by finding two members of $A\times B$ lying strictly between $\langle a_1,b_1\rangle$ and $\langle a_2,b_2\rangle$ in the product partial order on $A\times B$. You don’t have to look any further than the elements $a_1,a_2,b_1$, and $b_2$ to build these two members of $A\times B$.

• Show that if neither ($a_1=a_2$ and $b_1\prec b_2$) nor ($a_1\prec a_2$ and $b_1=b_2$) is true, then we’re in one of the three cases above. Conclude that since they’re impossible, we must have either ($a_1=a_2$ and $b_1\prec b_2$) or ($a_1\prec a_2$ and $b_1=b_2$).

From this result it follows that it is not true that $\langle a_1,b_1\rangle\prec\langle a_2,b_2\rangle$ if and only if $a_\prec a_2$ and $b_1\prec b_2$, which is what your sentence starting with And seems to claim.

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Thanks, I figured it out :) –  Edwardo Oct 1 '13 at 3:39
@Edwardo: Excellent! You’re welcome. –  Brian M. Scott Oct 1 '13 at 4:09