Not All Multi-Valued Partial CFL Functions Are Refined by Single-Valued   Functions
  Multi-valued partial CFL functions are functions computed along accepting computation paths by one-way nondeterministic pushdown automata, equipped with write-only output tapes, which are allowed to reject an input, in comparison with single-valued partial CFL functions. We give an answer to a fundamental question, raised by Konstantinidis, Santean, and Yu [Act. Inform. 43 (2007) 395-417], of whether all such multi-valued partial CFL functions can be refined by single-valued partial CFL functions. We negatively solve this open question by presenting a special multi-valued partial CFL function as an example function and by proving that no refinement of this particular function becomes a single-valued partial CFL function. This contrasts an early result of Kobayashi [Inform. Control 15 (1969) 95-109] that multi-valued partial NFA functions are always refined by single-valued NFA functions, where NFA functions are computed by one-way nondeterministic finite automata with output tapes. Our example function turns out to be unambiguously 2-valued, and thus we obtain a stronger separation result, in which no refinement of unambiguously 2-valued partial CFL functions can be single-valued. For the proof of this fact, we first introduce a new concept of colored automata having no output tapes but having "colors," which can simulate pushdown automata equipped with constant-space output tapes. We then conduct an extensive combinatorial analysis on the behaviors of transition records of stack contents (called stack histories) of these colored automata. 
Tomoyuki Yamakami
cs.FL cs.CC