How to substantiate the claim that something is not known? 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?
One way, especially since you are not an expert of the field (and even if you were), is to not make such an absolute claim:  


  To the best of our knowledge, no other necessary or sufficient
  conditions are currently known.


At least in my field (robotics), this is quite common and I think an appreciated amount of humility.