Variation of a tensor expression with indices

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SUMMARY

The discussion focuses on the variation of a tensor expression involving indices, specifically the expression ##\delta \bigg( \sqrt{- \eta_{\mu \nu} \frac{dx^{\mu}}{d \tau} \frac{dx^{\nu}}{d \tau}} \bigg)##. A participant suggests an incorrect manipulation of the expression by breaking out terms, which obscures the summation. The correct approach involves applying the chain rule, expressed as ##\delta f(x) = f'(x) \delta x##, to maintain clarity in the variation process.

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  • Knowledge of variational principles in physics
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spaghetti3451
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Say I want to find ##\delta \bigg( \sqrt{- \eta_{\mu \nu} \frac{dx^{\mu}}{d \tau} \frac{dx^{\nu}}{d \tau}} \bigg)##.

Is the following alright: ##\delta \bigg( \sqrt{- \eta_{\mu \nu}} \bigg( \frac{dx^{\mu}}{d \tau} \bigg)^{-1/2} \bigg( \frac{dx^{\nu}}{d \tau} \bigg)^{1/2} \bigg)##?
 
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failexam said:
Say I want to find ##\delta \bigg( \sqrt{- \eta_{\mu \nu} \frac{dx^{\mu}}{d \tau} \frac{dx^{\nu}}{d \tau}} \bigg)##.

Is the following alright: ##\delta \bigg( \sqrt{- \eta_{\mu \nu}} \bigg( \frac{dx^{\mu}}{d \tau} \bigg)^{-1/2} \bigg( \frac{dx^{\nu}}{d \tau} \bigg)^{1/2} \bigg)##?

No. You have broken out terms of a sum and it is no longer clear what is being summed with what. I suggest you use the chain rule ##\delta f(x) = f'(x) \delta x##.
 
Thanks! I've got it.
 

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