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I am learning QFT in the Peskin/Schroeder book and I found 4.56 on page 98 really weird, it is:

[tex]\rho_{vaccum\: energy\: density} = \frac{i\sum_{all\: disconnected\: diagramms}amplitude}{(2\pi)^4\delta^{(4)}(0)}[/tex]

The authors do not comment really this result, but could someone tell me at least if this is finite, calculable, ...

Thank you.