Question: Nonlinear system (has anyone seen this eq form before?)

nonlinear_system.pdf

Has anyone seen this form of a nonlinear equation with respect to X, but linear with respect to Y & Z?  I provided a contour plot within the region for all 3 variables between -2 & 2.  The plot is actually Z*conjugate(Z) so that the magnitude is above ZERO.  If I am correct I may have seen this before in orbital mechanics, but my undergrad years are a bit behind me.  I generated this eq. during a parametric analysis of a 2nd order oscillator.  I would like to come up to speed about the physical phenomena this equation might describe.

 

Appreciate any feedback!

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