Computer Science 456/656: Automata and Formal Languages

Spring 1999

Example 3 Answer

w_3 is not in the language L_d because it halts with any input file which has at least one "a" in it, and that includes itself.

Theoretically, you can prove that it halts by running it. But in practice you can't, because the required space is beyond the capacity of any machine that would fit into the known universe, and the required time exceeds the estimated remaining lifetime of the universe.

There is a quite simple proof that it halts that does not require emulation of the program. Can you prove it?

Back to Course Page