# Universal Turing Machine: Algorithmic Halting A natural number x will be identified with the x’th binary string in lexicographic order (Λ,0,1,00,01,10,11,000…), and a set X of natural numbers will be identified with its characteristic sequence, and with the real number between 0 and 1 having that sequence as its dyadic expansion. The length of a string x will be denoted |x|, the n’th bit of an infinite sequence X will be noted X(n), and the initial n bits of X will be denoted Xn. Concatenation of strings p and q will be denoted pq.

We now define the information content (and later the depth) of finite strings using a universal Turing machine U. A universal Turing machine may be viewed as a partial recursive function of two arguments. It is universal in the sense that by varying one argument (“program”) any partial recursive function of the other argument (“data”) can be obtained. In the usual machine formats, program, data and output are all finite strings, or, equivalently, natural numbers. However, it is not possible to take a uniformly weighted average over a countably infinite set. Chaitin’s universal machine has two tapes: a read-only one-way tape containing the infinite program; and an ordinary two-way read/write tape, which is used for data input, intermediate work, and output, all of which are finite strings. Our machine differs from Chaitin’s in having some additional auxiliary storage (e.g. another read/write tape) which is needed only to improve the time efficiency of simulations.

We consider only terminating computations, during which, of course, only a finite portion of the program tape can be read. Therefore, the machine’s behavior can still be described by a partial recursive function of two string arguments U(p, w), if we use the first argument to represent that portion of the program that is actually read in the course of a particular computation. The expression U (p, w) = x will be used to indicate that the U machine, started with any infinite sequence beginning with p on its program tape and the finite string w on its data tape, performs a halting computation which reads exactly the initial portion p of the program, and leaves output data x on the data tape at the end of the computation. In all other cases (reading less than p, more than p, or failing to halt), the function U(p, w) is undefined. Wherever U(p, w) is defined, we say that p is a self-delimiting program to compute x from w, and we use T(p, w) to represent the time (machine cycles) of the computation. Often we will consider computations without input data; in that case we abbreviate U(p, Λ) and T(p, Λ) as U(p) and T(p) respectively.

The self-delimiting convention for the program tape forces the domain of U and T, for each data input w, to be a prefix set, that is, a set of strings no member of which is the extension of any other member. Any prefix set S obeys the Kraft inequality

p∈S 2−|p| ≤ 1

Besides being self-delimiting with regard to its program tape, the U machine must be efficiently universal in the sense of being able to simulate any other machine of its kind (Turing machines with self-delimiting program tape) with at most an additive constant constant increase in program size and a linear increase in execution time.

Without loss of generality we assume that there exists for the U machine a constant prefix r which has the effect of stacking an instruction to restart the computation when it would otherwise end. This gives the machine the ability to concatenate programs to run consecutively: if U(p, w) = x and U(q, x) = y, then U(rpq, w) = y. Moreover, this concatenation should be efficient in the sense that T (rpq, w) should exceed T (p, w) + T (q, x) by at most O(1). This efficiency of running concatenated programs can be realized with the help of the auxiliary storage to stack the restart instructions.

Sometimes we will generalize U to have access to an “oracle” A, i.e. an infinite look-up table which the machine can consult in the course of its computation. The oracle may be thought of as an arbitrary 0/1-valued function A(x) which the machine can cause to be evaluated by writing the argument x on a special tape and entering a special state of the finite control unit. In the next machine cycle the oracle responds by sending back the value A(x). The time required to evaluate the function is thus linear in the length of its argument. In particular we consider the case in which the information in the oracle is random, each location of the look-up table having been filled by an independent coin toss. Such a random oracle is a function whose values are reproducible, but otherwise unpredictable and uncorrelated.

Let {φAi (p, w): i = 0,1,2…} be an acceptable Gödel numbering of A-partial recursive functions of two arguments and {φAi (p, w)} an associated Blum complexity measure, henceforth referred to as time. An index j is called self-delimiting iff, for all oracles A and all values w of the second argument, the set { x : φAj (x, w) is defined} is a prefix set. A self-delimiting index has efficient concatenation if there exists a string r such that for all oracles A and all strings w, x, y, p, and q,if φAj (p, w) = x and φAj (q, x) = y, then φAj(rpq, w) = y and φAj (rpq, w) = φAj (p, w) + φAj (q, x) + O(1). A self-delimiting index u with efficient concatenation is called efficiently universal iff, for every self-delimiting index j with efficient concatenation, there exists a simulation program s and a linear polynomial L such that for all oracles A and all strings p and w, and

φAu(sp, w) = φAj (p, w)

and

ΦAu(sp, w) ≤ L(ΦAj (p, w))

The functions UA(p,w) and TA(p,w) are defined respectively as φAu(p, w) and ΦAu(p, w), where u is an efficiently universal index.

For any string x, the minimal program, denoted x∗, is min{p : U(p) = x}, the least self-delimiting program to compute x. For any two strings x and w, the minimal program of x relative to w, denoted (x/w)∗, is defined similarly as min{p : U(p,w) = x}.

By contrast to its minimal program, any string x also has a print program, of length |x| + O(log|x|), which simply transcribes the string x from a verbatim description of x contained within the program. The print program is logarithmically longer than x because, being self-delimiting, it must indicate the length as well as the contents of x. Because it makes no effort to exploit redundancies to achieve efficient coding, the print program can be made to run quickly (e.g. linear time in |x|, in the present formalism). Extra information w may help, but cannot significantly hinder, the computation of x, since a finite subprogram would suffice to tell U to simply erase w before proceeding. Therefore, a relative minimal program (x/w)∗ may be much shorter than the corresponding absolute minimal program x∗, but can only be longer by O(1), independent of x and w.

A string is compressible by s bits if its minimal program is shorter by at least s bits than the string itself, i.e. if |x∗| ≤ |x| − s. Similarly, a string x is said to be compressible by s bits relative to a string w if |(x/w)∗| ≤ |x| − s. Regardless of how compressible a string x may be, its minimal program x∗ is compressible by at most an additive constant depending on the universal computer but independent of x. [If (x∗)∗ were much smaller than x∗, then the role of x∗ as minimal program for x would be undercut by a program of the form “execute the result of executing (x∗)∗.”] Similarly, a relative minimal program (x/w)∗ is compressible relative to w by at most a constant number of bits independent of x or w.

The algorithmic probability of a string x, denoted P(x), is defined as {∑2−|p| : U(p) = x}. This is the probability that the U machine, with a random program chosen by coin tossing and an initially blank data tape, will halt with output x. The time-bounded algorithmic probability, Pt(x), is defined similarly, except that the sum is taken only over programs which halt within time t. We use P(x/w) and Pt(x/w) to denote the analogous algorithmic probabilities of one string x relative to another w, i.e. for computations that begin with w on the data tape and halt with x on the data tape.

The algorithmic entropy H(x) is defined as the least integer greater than −log2P(x), and the conditional entropy H(x/w) is defined similarly as the least integer greater than − log2P(x/w). Among the most important properties of the algorithmic entropy is its equality, to within O(1), with the size of the minimal program:

∃c∀x∀wH(x/w) ≤ |(x/w)∗| ≤ H(x/w) + c

The first part of the relation, viz. that algorithmic entropy should be no greater than minimal program size, is obvious, because of the minimal program’s own contribution to the algorithmic probability. The second half of the relation is less obvious. The approximate equality of algorithmic entropy and minimal program size means that there are few near-minimal programs for any given input/output pair (x/w), and that every string gets an O(1) fraction of its algorithmic probability from its minimal program.

Finite strings, such as minimal programs, which are incompressible or nearly so are called algorithmically random. The definition of randomness for finite strings is necessarily a little vague because of the ±O(1) machine-dependence of H(x) and, in the case of strings other than self-delimiting programs, because of the question of how to count the information encoded in the string’s length, as opposed to its bit sequence. Roughly speaking, an n-bit self-delimiting program is considered random (and therefore not ad-hoc as a hypothesis) iff its information content is about n bits, i.e. iff it is incompressible; while an externally delimited n-bit string is considered random iff its information content is about n + H(n) bits, enough to specify both its length and its contents.

For infinite binary sequences (which may be viewed also as real numbers in the unit interval, or as characteristic sequences of sets of natural numbers) randomness can be defined sharply: a sequence X is incompressible, or algorithmically random, if there is an O(1) bound to the compressibility of its initial segments Xn. Intuitively, an infinite sequence is random if it is typical in every way of sequences that might be produced by tossing a fair coin; in other words, if it belongs to no informally definable set of measure zero. Algorithmically random sequences constitute a larger class, including sequences such as Ω which can be specified by ineffective definitions.

The busy beaver function B(n) is the greatest number computable by a self-delimiting program of n bits or fewer. The halting set K is {x : φx(x) converges}. This is the standard representation of the halting problem.

The self-delimiting halting set K0 is the (prefix) set of all self-delimiting programs for the U machine that halt: {p : U(p) converges}.

K and K0 are readily computed from one another (e.g. by regarding the self-delimiting programs as a subset of ordinary programs, the first 2n bits of K0 can be recovered from the first 2n+O(1) bits of K; by encoding each n-bit ordinary program as a self-delimiting program of length n + O(log n), the first 2n bits of K can be recovered from the first 2n+O(log n) bits of K0.)

The halting probability Ω is defined as {2−|p| : U(p) converges}, the probability that the U machine would halt on an infinite input supplied by coin tossing. Ω is thus a real number between 0 and 1.

The first 2n bits of K0 can be computed from the first n bits of Ω, by enumerating halting programs until enough have halted to account for all but 2−n of the total halting probability. The time required for this decoding (between B(n − O(1)) and B(n + H(n) + O(1)) grows faster than any computable function of n. Although K0 is only slowly computable from Ω, the first n bits of Ω can be rapidly computed from the first 2n+H(n)+O(1) bits of K0, by asking about the halting of programs of the form “enumerate halting programs until (if ever) their cumulative weight exceeds q, then halt”, where q is an n-bit rational number…