 
Summary: Nondeterministic Streaming String Transducers
Rajeev Alur and Jyotirmoy V. Deshmukh
Dept. of Computer and Information Science,
University of Pennsylvania.
{alur,djy}@cis.upenn.edu
Abstract. We introduce nondeterministic streaming string transducers
(nssts) a new computational model that can implement MSOdefinable
relations between strings. An nsst makes a single lefttoright pass on
the input string and uses a finite set of string variables to compute the
output. In each step, it reads one input symbol, and updates its string
variables in parallel with a copyless assignment. We show that nsst are
closed under sequential composition and that their expressive power co
incides with that of nondeterministic MSOdefinable transductions. Fur
ther, we identify the class of functional nssts; such an nsst allows non
deterministic transitions, but for every successful run on a given input
generates the same output string. We show that deciding functionality of
an arbitrary nsst is decidable with pspace complexity, while the equiv
alence problem for functional nssts is pspacecomplete. We also show
that checking if the set of outputs of an nsst is contained within the set
of outputs of a finite number of dssts is decidable in pspace.
