A criterion for complete lattice.

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Is there an infinite partially ordered set $(X,\le)$, in which for each $A\subseteq X$, either $\inf A$ or $\sup A$ exists but for some $A\subseteq X$ either $\inf A$ or $\sup A$ does not exist.

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Sure. Take the order type $\omega$. (To be clear: Here $\emptyset$ has a supremum but not an infimum, and the whole set has an infimum but not a supremum.)