1. Theory of Computability
Giorgi Japaridze
Theory of Computability
Reducibility
Chapter 5

2. The undecidability of the halting problem
Giorgi Japaridze Theory of Computability
Let HALTTM = {<M,w> | M is a TM and M halts on input w}
HALTTM is called the halting problem.
Theorem 5.1: HALTTM is undecidable.
Proof idea: Assume, for a contradiction, that HALTTM is decidable. I.e.
there is a TM R that decides HALTTM. Construct the following TM S:
S = “On input <M,w>, an encoding of a TM M and a string w:
1. Run R on input <M,w>.
2. If R rejects, reject.
3. If R accepts, simulate M on w until it halts.
4. If M has accepted, accept; if M has rejected, reject.”
If M works forever on w, what will S do on <M,w>?
If M accepts w, what will S do on input <M,w>?
If M explicitly rejects w, what will S do on <M,w>?
Thus, S decides the language
But this is impossible (Theorem 4.11)

3. Definition of mapping reducibility
Giorgi Japaridze Theory of Computability
We say that A is
mapping reducible to B, written AmB, if there is a computable
function f: ** such that, for every w*,
wA iff f(w)B.
The function f is called a mapping reduction of A to B.
Let A and B be languages over an alphabet . * * A B f f

4. An example of a mapping reduction
5.3.b
Giorgi Japaridze Theory of Computability
Let f be the function computed by the following TMO M:
M=“On input <N,w>, where N is an NFA and w is a string,
1. Convert N into an equivalent DFA D using the algorithm
we learned;
2. Return <D,w>.”
f is then a mapping reduction of what language to what language? * *
<N,w>
<D,w>
<D,w>
<N,w>

5. Using mapping reducibility for proving decidability/undecidability
Giorgi Japaridze Theory of Computability
Theorem 5.22: If AmB and B is decidable, then A is decidable.
Proof: Let DB be a decider for B and f be a reduction from A to B.
We describe a decider DA for A as follows.
DA= “On input w:
1. Compute f(w).
2. Run DB on input f(w) and do whatever DB does.”
Corollary 5.23: If AmB and A is undecidable, then B is undecidable.
Theorem remains valid with “Turing recognizable” instead of
“decidable”. So does Corollary 5.23.

6. A mapping reduction of ATM to HALTTM
Giorgi Japaridze Theory of Computability
For every TM M, let M* be the following TM:
M* = “On input x:
1. Run M on x.
2. If M accepts, accept.
3. If M rejects, enter an infinite loop.”
Thus,
If M accepts input x, then M*
If M explicitly rejects x, then M*
If M never halts on x, then M*
To summarize, M accepts x iff M*
Let then f be the function defined by f(<M,w>)=<M*,w>.
Is f computable?
Obviously <M,w>ATM iff f(<M,w>)
i.e. f is a
So, since ATM is undecidable, HALTTM is undecidable as well.