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I understand the classical Lagrangian which follows the Principle of Least Action(A)

A=∫L dt

But what is Lagrangian density? Is it a new concept?

A=∫Lagrangian density dx^4

Here 4 is the four vector? One time-like and 3 space-like co-ordinates?

QFT uses Lagrangian to field theory?

Thanks.

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# Lagrangian and quantum field theory

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