Sentence examples for been a difficult problem from inspiring English sources

Reliably forecasting the timing, location, and magnitude of an earthquake has been a difficult problem to crack.

Multi-objective optimization has been a difficult problem and a research focus in the field of science and engineering.

We established a method to obtain reliable I and τi by adding NA which has been a difficult problem in polymer science.

Surface bughole of concrete is one of surface imperfections of concrete appearance, and the detection and evaluation of surface bugholes have always been a difficult problem.

The accuracy verification of catenary dynamic model has been a difficult problem in this field, and the corresponding standards for the correctness of the simulation model have not been established in China.

The existence of altruistic behaviors has been a difficult problem for evolutionary theorists from the beginning.

It is a difficult problem.

"That's a difficult problem".

It's a difficult problem.

How to do it is a difficult problem.

Reinvigorating democratic political competition is a difficult problem.