There are still many open problems in this area.

If this is an optimal construction remains an open problem.

In fact, each of the following is an open problem:

However, there has been no progress on the name issue which remains an open problem.

The existence of such an example was an open problem for many years.

The exact value of the sofa constant is an open problem.

It is common in graduate schools to point out open problems to students.

It is an open problem as to whether such constructions are possible.

There, he was the first put forth the question of what shapes are possible as an open problem.

Though they have been studied for more than a hundred years, many open problems remain.