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I have a question about the most general (diff$\times$Weyl)-invariant and Poincare-invariant form of action. In Polchinski's string theory p15, there is an action for manifold without boundary $$S_{\mathrm{p}}'= - \int_M d \tau d \sigma (-\gamma)^{1/2} \left( \frac{1}{4\pi \alpha'} \gamma^{ab} \partial_a X^{\mu} \partial_b X_{\mu} + \frac{ \lambda}{4\pi} R \right) \tag{1.2.33}$$

It is said

This is the most general (diff$\times$Weyl)-invariant and Poincare-invariant action with these fields and symmetries.

My question is, how to prove Eq. (1.2.33) is the most general one (without boundary)? Not saying what else could it be...

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