## Every 2-random real is Kolmogorov random.(English)Zbl 1090.03012

A binary sequence $$\omega$$ is Martin-Löf random if all effectively checkable probability laws (such as the law of large numbers) hold for this sequence, i.e., if $$\omega$$ does not belong to any effective set of measure 0. Another reasonable definition of randomness, called Kolmogorov randomness, generalizes Kolmogorov’s idea that a finite sequence $$\omega$$ of length $$n$$ can be called random if it cannot be algorithmically compressed, i.e., if its Kolmogorov complexity $$K(\omega)$$ (the shortest length of a program that computes $$\omega$$) is $$\geq n-C$$ for some small constant $$C$$. It is therefore natural to describe an infinite sequence $$\omega=\omega_1\omega_2\ldots$$ as Kolmogorov random if, for infinitely many $$n$$, we have $$K(\omega_1\ldots\omega_n)\geq n-O(1)$$. It is known that every Kolmogorov random sequence is Martin-Löf random but there are Martin-Löf random sequences which are not Kolmogorov random.
The author provides a new equivalent characterization of Kolmogorov randomness in Martin-Löf-type terms. Specifically, in Martin-Löf’s definition, we limit ourselves to effectively checkable probability laws – i.e., laws of of the type $$\forall n P$$. For such laws, the measure 0 set of all the sequences that do not satisfy this law has the form $$\exists n \neg P$$, i.e., the form $$\Sigma_1^0$$. Since the fact that a sequence $$\omega$$ satisfies all effective laws is not sufficient to imply its non-compressibility, it is reasonable to require that $$\omega$$ should satisfy more general probability laws as well – e.g., laws for which the measure 0 set is described by $$\Sigma^0_2$$ formulas $$\exists n\forall m P$$. It turns out that the resulting “2-randomness” is indeed equivalent to Kolmogorov randomness.

### MSC:

 03D80 Applications of computability and recursion theory 68Q30 Algorithmic information theory (Kolmogorov complexity, etc.)
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### References:

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