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I am looking for a way to prove that a given LTL formula is expressed with the fewest number of temporal operators possible.

I would like to do this to compare the expressive length with other temporal logic languages like MTL.

How can I prove that a given LTL formula is expressed most succinctly as possible? So as to say, this formula needs at least this many temporal operators.

For example, suppose I want to code that property p holds in every other state for the next 6 states starting from the current state, then I could specify this as, $p \wedge XXp \wedge XXXXp \wedge XXXXXXp = p \wedge XX(p \wedge XX(p \wedge XXp))$, where $X$ is the next operator, but how do I prove that this the smallest way to express this?

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I'm not sure whether it would work in your case, but in order to show succinctness results in modal/temporal logic (e.g. the fact the two-variable logic over words in exponentially more succinct than unary temporal logic) one can employ formula size games or Adler-Immerman games.

Probably the most recent paper to read is by Lauri Hella and Miikka Vilander. Its free-access version is available on arxiv here.

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Your question is formally the same as this one: how many symbols $X$ are needed to write the polynomial $XX + XXXX + XXXXXX = XX(1 + XX(1 + XX))$? Well, you obtained $6$ as an upper bound. It is also a lower bound because any expression involving less than $6$ symbols $X$ would define a polynomial of degree $< 6$.

For the general case, I would not be surprised if it turns out to be a NP-complete problem.

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