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Gaussian97

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- I've just read that for a Lagrangian to be Lorentz Invariant the Lagrangian density cannot have second or higher derivatives.

Last day in class, a professor told us that, for a Lagrangian to be Lorentz Invariant, the Lagrangian density cannot have second or higher derivatives. Is this true?

Because one can write the KG lagrangian as $$\mathscr{L}=\phi(\square + m^2)\phi,$$ which have second derivatives.

And, where can I find a proof of this fact?

Thank you

Because one can write the KG lagrangian as $$\mathscr{L}=\phi(\square + m^2)\phi,$$ which have second derivatives.

And, where can I find a proof of this fact?

Thank you