I am writing a master's thesis (in Number Theory) and there are multiple places where I need to claim that something is not yet known. Since I am not an expert (and even if I were) I would like to reference some more authoritative source than myself or, say, Wikipedia.

For example, consider the following statement:

In 1980 Schmidt proved that [some property holds]. No other necessary or sufficient conditions are currently known, though.

For the first sentence I can cite Schmidt's original paper, but how can I substantiate the second claim?