Ah, mixed derivatives![$]\frac{\partial^2 \, \mbox{Area of a circle}}{\partial \pi \partial x}=ab[$]I often explain vega via

[$]\frac{\partial \, \mbox{Area of a circle}}{\partial \pi}=r^2[$]

this because a circle moved along the x axis is observed to be an ellipse due to length contraction, so we have the well unknown circle equation

[$]\frac{\partial^2 \, \mbox{Area of a circle}}{\partial \pi \partial t}-\frac{\partial^2 \, \mbox{Area of a circle}}{\partial \pi \partial x}=r^2-ab[$]

or more interesting

[$]\frac{\partial \, \mbox{Area of a circle}}{\partial t}-\frac{\partial \, \mbox{Area of a circle}}{ \partial x}=\pi r^2-\pi ab[$]

where a and b functions of v

and do we also get:?

[$]\frac{\partial \, \mbox{Area of a circle}}{\partial t}-\nabla \mbox{Area of a circle} =\pi r^2-\pi ab[$]

more complex under acceleration.

Would they be called "mixed Greeks"?