The Navier–Stokes equations have a general form that can take into account many physical phenomena including the difference between volume…

The Navier–Stokes equations have a general form that can take into account many physical phenomena including the difference between volume viscosity and dynamic viscosity, compressible flow, change of viscosities in the flow, and an external force. The Millennium Prize is offered for proving any one of four statements, two of which deal with the Navier–Stokes equations with no external force while the other two deal with the Navier–Stokes equations with a smooth external force.

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Source: List of unsolved problems in mathematics (CC BY-SA 4.0)