Has anyone come across this operator ?(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\frac{\partial^2}{\partial_x\partial_y} + \frac{\partial^2}{\partial_x\partial_z} + \frac{\partial^2}{\partial_z\partial_y}[/tex]

I've never seen it until it came up in a field theory context. What can it mean ?

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# Mystery (?) operator

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