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I've got a question about a term in a formula I've found in Mandl&Shaw's QFT book

It's about equation 2.18 on page 31

[tex] L(t) = {\sum_i \delta \bf{x}_i {\cal L}_i \ ... [/tex]

Why is there a delta x_i when summing over all lagrangians for getting the lagrange-function for the whole system?

And what operator is that delta in this particular equation?

The difference between two different points? (wouldn't that rather be [tex] \Delta \bf{x} [/tex] ?)

[tex] \delta [/tex] serves only as functional derivative AFAIK

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# Explanation for term in formula, QFT

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