The following are a few basic concepts of the theory of Kolmogorov complexity (see the Appendix for formal details):
Compiler theorem. Informally, this theorem says that any program for a given computer can be compiled into an equivalent program for a given universal computer by a compiler program whose length does not depend on the programs it compiles.
Kolmogorov complexity. The Kolmogorov complexity of a computable object is the length of the shortest program that computes it on a universal computer and halts.
Invariance theorem. Essentially, the invariance theorem says that the Kolmogorov complexity of some object does not depend of the particular computer used, leaving aside an additive, machine-specific, object-independent constant. This objectivity is due to the compiler theorem.
Noncomputability of Kolmogorov complexity. It can be shown that there is no single algorithm that can generate the shortest program for computing arbitrary given data on a given computer .