Wave particle duality

Background.

What are (quantum mechanical) entities, either a particle or a wave or both or none?

Methods.

The most elementary and simple rules of the special relativity theory were used to approach the solution of this matter.

Results.

The particle wave duality has been mathematised very precisely. A relativistic wave equation has been developed.

Conclusion.

Under usual circumstances, a (quantum mechanical) entity is equally both, a particle and a wave.