Compression theorem

In computational complexity theory the compression theorem is an important theorem about the complexity of computable functions.

The theorem states that there exists no largest complexity class, with computable boundary, which contains all computable functions.

Compression theorem

Given a Gödel numbering $\varphi$ of the computable functions and a Blum complexity measure $\Phi$ where a complexity class for a boundary function $f$ is defined as

\mathrm{C}(f):= \{\varphi_i \in \mathbf{R}^{(1)} | (\forall^\infty x) \, \Phi_i (x) \leq f(x) \}.

Then there exists a total computable function $f$ so that for all $i$

\mathrm{Dom}(\varphi_i) = \mathrm{Dom}(\varphi_{f(i)})

and

\mathrm{C}(\varphi_i) \subsetneq \mathrm{C}(\varphi_{f(i)}).

References

Salomaa, Arto (1985), "Theorem 6.9", Computation and Automata, Encyclopedia of Mathematics and Its Applications 25, Cambridge University Press, pp. 149–150, ISBN 9780521302456 .
Zimand, Marius (2004), "Theorem 2.4.3 (Compression theorem)", Computational Complexity: A Quantitative Perspective, North-Holland Mathematics Studies 196, Elsevier, p. 42, ISBN 9780444828415 .