# A Theory of Program Size Formally Identical to Information Theory

## Abstract

A new definition of program-size complexity is made. H(A,B/C,D) is defined to be the size in bits of the shortest self-delimiting program for calculating strings A and B if one is given a minimal-size self-delimiting program for calculating strings C and D. This differs from previous definitions: (1) programs are required to be self-delimiting, i.e. no program is a prefix of another, and (2) instead of being given C and D directly, one is given a program for calculating them that is minimal in size. Unlike previous definitions, this one has precisely the formal properties of the entropy concept of information theory. For example, H(A,B) = H(A) + H(B/A) -0(1). Also, if a program of length k is assigned measure 2 -k, then H(A) = -log2 (the probability that the standard universal computer will calculate A) -{- 0(1). © 1975, ACM. All rights reserved.