"problem under investigation" is a correct and usable phrase in written English.
You can use it to refer to an issue that someone is trying to resolve or find an answer to. Example: The police are looking into the murder, but the exact motive remains a problem under investigation.
In our case, the problem under investigation is (2 + 1 -nonlinear partial differential equations (PDEs).
The second difficulty raised by the problem under investigation is the discrete nature of channel assignments in the scheduling policy.
The problem under investigation here is that of estimating the fundamental frequencies of periodic signals in noise.
In a first attempt, we cast the problem under investigation immediately into the SIS algorithm by defining.
To begin with this concerns the generality of the conditions of input data of the problem under investigation.
In the inverse problem under investigation the location and strength of the line heat sources are unknown.
Joan P. Platt, the chief financial officer at Indus in 1999, said in a brief telephone interview that the use of side letters by the company's sales force -- without the knowledge of company accountants -- caused the revenue-recognition problems under investigation.
Environmental monitoring results therefore have to be unambiguous to avoid misunderstanding the problems under investigation.
It has become obvious that finding the genes by either differential display or DNA microarray is only the first step toward the understanding of biological problems under investigation.
But given the complexity of the problems under investigation, communicating insights from research to a broader public will always present a challenge.
Beyond the direct interest, some other scientific problems under investigation can be also transformed into this similar issue.
