Computability and Logic

94 EQUIVALENT DEFINITIONS OF COMPUTABILITY
8.2 Universal Turing Machines
The connection we have established between Turing computability and recursiveness
enables us to establish properties of each notion that it would have been more difficult
to establish working with that notion in isolation. We begin with one example of this
phenomenon pertaining to Turing machines, and one to recursive functions.
8.3 Theorem. The same class of functions are Turing computable whether one defines
Turing machines to have a tape infinite in both directions or infinite in only one direction,
and whether one requires Turing machines to operate with only one symbol in addition to
the blank, or allows them to operate with any finite number.
Proof: Suppose we have a Turing machine M of the kind we have been working
with, with a two-way infinite tape. In this chapter we have seen that the total or
partial function f computed by M is recursive. In earlier chapters we have seen how
a recursive function f can be computed by an abacus machine and hence by a Turing
machine simulating an abacus machine. But the Turing machines simulating abacus
machines are rather special: according to the problems at the end of Chapter 5, any
abacus-computable function can be computed by a Turing machine that never moves
left of the square on which it is started. Thus we have now shown that for any Turing
machine there is another Turing machine computing the same function that uses only
the right half of its tape. In other words, if we had begun with a more restrictive notion
of Turing machine, where the tape is infinite in one direction only, we would have
obtained the same class of Turing-computable functions as with our official, more
liberal definition.
Inversely, suppose we allowed Turing machines to operate not only with the blank
0 and the stroke 1, but also with another symbol 2. Then in the proof of the preceding
sections we would need to work with ternary rather than binary numerals, to code
Turing machines by sequences of length a multiple of six rather than of four, and
make similar minor changes. But with such changes, the proof would still go through,
and show that any function computable by a Turing machine of this liberalized kind
is still recursive—and therefore was computable by a Turing machine of the original
kind already. The result generalizes to more than two symbols in an obvious way:
for n symbols counting the blank, we need n-ary numerals and sequences of length a
multiple of 2n.
Similar, somewhat more complicated arguments show that allowing a Turing machine
to work on a two-dimensional grid rather than a one-dimensional tape would
not enlarge the class of functions that are computable. Likewise the class of functions
computable would not be changed if we allowed the use of blank, 0, and 1, and redefined
computations so that inputs and outputs are to be given in binary rather than
stroke notation. That class is, as is said, stable under perturbations of definition, one
mark of a natural class of objects.
8.4 Theorem (Kleene normal form theorem). Every recursive total or partial function
can be obtained from the basic functions (zero, successor, identity) by composition, primitive
recursion, and minimization, using this last process no more than once.