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# Tree automata

## Finite Automata as Automata in Unary Language

Consider a finite alphabet . A word is a finite sequence of symbols from .

We can represent such a sequence as a term, as follows

• treat elements of as unary function symbols
• let be a constant symbol

Represent as a ground term

We define the notion of finite state machine as accepting a set of such terms.

## Bottom up tree automata

Definition:Given an alphabet , a non-deterministic bottom-up tree automaton in language is where

• is a set of states
• is the set of final states
• is a set of rewrite rules

where is a function symbol of arity and where .

Note: In the case of constant the transition has the form and specifies possible initial states.

A ground term in language is accepted by the automaton iff it is possible to reach by apply rewrite rules in to term .

This rewriting process is a sequence of transitions where are terms in language with being treated as constants of arity zero.

Definition: A regular tree language is a set of terms accepted by a non-deterministic bottom-up tree automaton.

Example:

• bottom up tree automaton accepting trees with an even number of nodes in the tree, in alphabet with constant 'a', binary function 'f'.
• bottom up tree automaton accepting expressions with complement, union, intersection; accepting only expressions that are monotonic in each variable

Non-deterministic top-down tree automaton: reverse a bottom-up tree automaton

Deterministic top-down tree automaton: more restrictive. Is there deterministic automaton to check the parity of the number of nodes?

## Closure Properties of Tree Automata

Definition: We say bottom-up tree automaton is deterministic iff for every of arity and there exists exactly one such that .

Lemma: For every bottom up tree automaton there exists a deterministic bottom up tree automaton accepting the same language.

Closure under:

• union
• negation
• intersection

Comparison to context-free grammars: closure under intersection and complement. Parser trees are regular, their yield is not.

Projection.

## Weak Monadic Logic of Two Successors (WS2S)

We generalize Weak Monadic Logic of One Successor to logic that allows quantification over finite subsets of an infinite binary tree.

• a finite subset of nodes in an infinite binary tree can be specified by listing, for each node in , a path in the binary that leads to this node
• a path in a binary tree is a finite sequence of “left” and “right” moves from the root
• we encode each node as a string

The domain of WS2S is the set of all finite subsets .

Minimalistic syntax:

We consider interpretations where

where takes a tree node and finds its left child:

and where takes a tree node and finds its right child:

The meaning of formulas is then given by standard First-order logic semantics.

## Deciding WS2S

A construction is similar to one for WS1S and is possible thanks to closure properties of finite automata.

Examples:

• subset
• successor

## Infinite Trees

S2S (without restriction to finite sets) is also decidable. The proofs are much more complex:

• cannot use bottom-up automata
• complementation is difficult

Key reference: