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• These equations can be used to construct the quasi-stationary idealization of curl: $$\partial p/\partial r=\frac{\rho}{r} u_\phi^2$$, an The Navier-Stokes equations follow from the [[principle of the stationary action]] for the Lagrangian
• ...some Lagrangian $$L=L(x,u(x),u_\bullet(x))$$, realizes the extremum of the action Then, the variation of the action