How do the interval and lower topologies relate when generated by an arbitrary poset?

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At first glance, it would seem that the lower topology (and the upper topology, for that matter) would be a subset of the interval topology for a partially ordered set P, since the open-ended intervals are the down-sets for a point p. What complications arise because P is a partial order instead of a total order? Is this the problem? Is it because the interval topology is not built up from down-sets containing incomparable elements? Any non-obvious counterexamples would be much appreciated. Thank you.

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Lots. Interval topology, if we're talking about the same thing, on total order is T5. On poset it is T1. This is mentioned at the start of the article* , maybe proofs, if you need them, are in references:

*SEPARATION AXIOMS FOR INTERVAL TOPOLOGIES