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Abstract
Weighted automata are widely researched, but with a variety of different semantics, which mostly fit into either the “quantitative view” or the “algebraic view”. We argue that the two views result with incomparable automata families, each providing a different conceptual generalization of Boolean automata and having different natural extensions.
We propose to term the former “quantitative automata” and the latter “weighted automata”.
In both views, transitions are labeled with weights and the value of a path of transitions is given by some value function on the traversed weights. However, the main conceptual difference is in the generalization of nondeterminism and its dual (universality, in alternating automata).
Quantitative automata keep the preference meaning of choice and obligation to nondeterminism and universality, interpreted as supremum and infimum, respectively, and accordingly restrict weights and value functions to the totally ordered domain of real numbers.
Weighted automata, on the other hand, generalize nondeterminism to an arbitrary commutative operation (of a semiring or valuation monoid), and generally have no interpretation of universality. The weights and value functions can be from arbitrary domains.
On several aspects the algebraic view generalizes the quantitative one, allowing for richer weight domains and interpretations of nondeterminism, whereas on different aspects the quantitative view is more general, having alternation, inherent compatibility with games and adequacy to approximations.
We argue that clarifying the conceptual difference between the two automata families can enlighten their possible future extensions.
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We define first an automaton without an acceptance-condition/value-function/semi-ring/valuation-monoid, but with initial state(s). Usually, the term ‘automaton’ refers to an entity with them, while ‘semiautomaton’ to an entity that also lacks initial state(s).
This extra flexibility of allowing for “parallel” transitions with different weights is often omitted, since it is redundant for some value functions while important for others.
There are value functions that are more naturally defined over sequences of tuples of real values (see Remark 1), for example lexicographic-mean-payoff [9] and discounted-summation with multiple discount factors [13].
It is sometimes defined analogously with infimum instead of supremum. Considering alternating quantitative automata, infimum relates to universal transitions.
Semiring weighted automata are sometimes defined with an acceptance condition (e.g., [48]) and sometimes without it, while having instead labels on both transitions and states (e.g., [34]). However, considering infinite words or valuation monoids, weighted automata have an acceptance condition [38].
For finite words, the acceptance condition is a set \(F\subseteq Q\) of final states, and a run satisfies it if it ends in a final state. For infinite words there are many acceptance conditions, such as Büchi or Muller. (More details on the different acceptance conditions can be found, for example, in [12]).
The “weighted automata” in the title of [49] refer to a variant of \({\mathsf {Sum}}\) automata, defined over infinite words with an acceptance condition on finite prefixes.
As ratio does not satisfy the triangle inequality, it is formally not a distance function, and one may speak instead of \(d(x,y) = |\log x - \log y|\).