STL Robustness Risk over Discrete-Time Stochastic Processes
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We present a framework to interpret signal temporal logic (STL) formulas over discrete-time stochastic processes in terms of the induced risk. Each realization of a stochastic process either satisfies or violates an STL formula. In fact, we can assign a robustness value to each realization that indicates how robustly this realization satisfies an STL formula. We then define the risk of a stochastic process not satisfying an STL formula robustly, referred to as the "STL robustness risk". In our definition, we permit general classes of risk measures such as, but not limited to, the value-at-risk. While in general hard to compute, we propose an approximation of the STL robustness risk. This approximation has the desirable property of being an upper bound of the STL robustness risk when the chosen risk measure is monotone, a property satisfied by most risk measures. Motivated by the interest in data-driven approaches, we present a sampling-based method for calculating an upper bound of the approximate STL robustness risk for the value-at-risk that holds with high probability. While we consider the case of the value-at-risk, we highlight that such sampling-based methods are viable for other risk measures.
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