Too add to what said, there is an nice and open free course on the edx platform that goes though the theory and which I would recommend :
Finally, we do have a theory section in the apdl help manual (chapter 2). Of course we can not expand on that (so if someone can not find something there, we can not provide additional comments), so we can only refer to that theory section. But still perhaps it is a useful reference for users.
As for the rigid body modes, they come out when solving the eigenvalue problem - this example illustrates that:
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