Many years ago I heard that computing the minimal NFA (nondeterministic finite automaton) from a DFA (deterministic) was an open question, as opposed to the vice versa direction which has been known for decades and is well researched with an efficient $O(n \lg n)$ algorithm. Has anyone come up with an algorithm?
A quick search gave me this paper that proves that its definitely a hard problem. Apparently, no algorithm is given.
[1] Minimal NFA problems are hard / Tao Jiang and B. RavikumarMinimal NFA problems are hard / Tao Jiang and B. Ravikumar
I was reminded of this problem by the following question on the CS.SE site for which a DFA->NFA minimization algorithm would be closely related. This following question seems to me to be research level. I suggested migrating it to TCS and I wrote an answer suggesting a statistical/empirical attack.
[2] What are the conditions for a NFA for its equivalent DFA to be maximal in size?