Upper semicomputable sumtests for lower semicomputable semimeasures

A sumtest for a discrete semimeasure P is a function f mapping bitstrings to non-negative rational numbers such that ∑ P (x)f(x) ≤ 1 . Sumtests are the discrete analogue of Martin-Löf tests. The behavior of sumtests for computable P seems well understood, but for some applications lower semicomputable P seem more appropriate. In the case of tests for independence, it is natural to consider upper semicomputable tests (see [B.Bauwens and S.Terwijn, Theory of Computing Systems 48.2 (2011): 247268]). In this paper, we characterize upper semicomputable sumtests relative to any lower semicomputable semimeasures using Kolmogorov complexity. It is studied to what extend such tests are pathological: can upper semicomputable sumtests form(x) be large? It is shown that the logarithm of such tests does not exceed log |x| + O(log |x|) (where |x| denotes the length of x and log = log log) and that this bound is tight, i.e. there is a test whose logarithm exceeds log |x| −O(log |x|) infinitely often. Finally, it is shown that for each such test e the mutual information of a string with the Halting problem is at least log e(x)−O(1); thus e can only be large for “exotic” strings.