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  1. The successes of reinforcement learning in recent years are underpinned by the characterization of suitable reward functions. However, in settings where such rewards are non-intuitive, difficult to define, or otherwise error-prone in their definition, it is useful to instead learn the reward signal from expert demonstrations. This is the crux of inverse reinforcement learning (IRL). While eliciting learning requirements in the form of scalar reward signals has been shown to be effective, such representations lack explainability and lead to opaque learning. We aim to mitigate this situation by presenting a novel IRL method for eliciting declarative learning requirements in the form of a popular formal logic---Linear Temporal Logic (LTL)---from a set of traces given by the expert policy. 
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    Free, publicly-accessible full text available May 30, 2024
  2. Chechik, M. ; Katoen, JP. ; Leucker, M. (Ed.)
    Efficient verification algorithms for neural networks often depend on various abstract domains such as intervals, zonotopes, and linear star sets. The choice of the abstract domain presents an expressiveness vs. scalability trade-off: simpler domains are less precise but yield faster algorithms. This paper investigates the octatope abstract domain in the context of neural net verification. Octatopes are affine transformations of n-dimensional octagons—sets of unit-two-variable-per-inequality (UTVPI) constraints. Octatopes generalize the idea of zonotopes which can be viewed as an affine transformation of a box. On the other hand, octatopes can be considered as a restriction of linear star set, which are affine transformations of arbitrary H-Polytopes. This distinction places octatopes firmly between zonotopes and star sets in their expressive power, but what about the efficiency of decision procedures? An important analysis problem for neural networks is the exact range computation problem that asks to compute the exact set of possible outputs given a set of possible inputs. For this, three computational procedures are needed: 1) optimization of a linear cost function; 2) affine mapping; and 3) over-approximating the intersection with a half-space. While zonotopes allow an efficient solution for these approaches, star sets solves these procedures via linear programming. We show that these operations are faster for octatopes than the more expressive linear star sets. For octatopes, we reduce these problems to min-cost flow problems, which can be solved in strongly polynomial time using the Out-of-Kilter algorithm. Evaluating exact range computation on several ACAS Xu neural network benchmarks, we find that octatopes show promise as a practical abstract domain for neural network verification. 
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    Free, publicly-accessible full text available March 3, 2024
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