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Tests for Manifolds?

SLD

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So, just playing around with a graphics program, and using spherical coordinates, phi, rho and r. I graphed this equatio:

D55D5E53-7AE9-4F3F-9E85-0E9DADA989C7.jpeg
here is the result:

60D6B95E-C62E-4433-8316-257898FD21D1.jpeg
Rather odd looking fu cation. The bumpiness is due to the limits of the program, but clearly part of the domain of this function is indeed a smooth manifold. But it doesn’t look like all of it is. Is there a mathematical way to determine whether it is Everywhere?

You can obviously differentiate it with respect to phi and rho. But just visually it doesn’t look like it’s a smooth manifold everywhere.

I’ve created a few other odd ones that I’m not sure about either.
 
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