A problem on partially ordered set

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Two different posets cannot have the same Hasse diagram, but they may have the same cover graph or the same comparability graph. How to prove the first one and show examples of the other cases.

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HINTS:

  • How you prove that two different posets cannot have the same Hasse diagram will depend a bit on how rigorous you’re supposed to be. You could suppose that $\langle P,\le\rangle$ and $\langle P,\preceq\rangle$ have the same Hasse diagram but that there are $p,q\in P$ such that $p\le q$ but $p\not\preceq q$. Can you see how to get a contradiction from this?

  • Consider the graph $G$ with two vertices and one edge; can you find two different posets that have $G$ as cover graph? How about two different posets that have $G$ as their comparability graph?