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It's a theorem that, although the Kolmogorov complexity of a string is relative to the Turing machine you're working with, it differs by at most a constant (basically the amount of space it takes to write an interpreter for Turing machine A inside of Turing machine B).

My question is: how does this theorem deal with the issue of Turing machines with purposefully inefficient encodings?

For example, suppose we have a universal Turing machine $M_{terrible}$ that requires its input to be repeated five times. It starts by verifying that the input is in fact of the form x ++ x ++ x ++ x ++ x, then executes the universal Turing machine $N$ on just x. Shouldn't this force the Kolmogorov complexity of strings relative to $M_{terrible}$ to be 5 times larger than they are relative to $N$? Why doesn't this violate the theorem?

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    $\begingroup$ The theorem is that there are computable essentially-optimal description languages. ​ ​ $\endgroup$ – user12859 Mar 18 '16 at 22:13
  • $\begingroup$ @RickyDemer Make an answer? $\endgroup$ – Yuval Filmus Mar 18 '16 at 22:47
  • $\begingroup$ @YuvalFilmus : ​ I get "Trivial answer converted to comment". ​ ​ ​ ​ $\endgroup$ – user12859 Mar 19 '16 at 3:25
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You are stating the theorem incorrectly. The theorem states that the Kolmogorov complexity is the same up to a constant for essentially optimal description languages, which are those in which you can describe running a Turing machine on an input $x$ using $|x| + O(1)$ bits. Given this definition, it is a simple matter of playing with definitions to see that all such description languages result in the same notion of Kolmogorov complexity, up to an additive constant.

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