[tex]\frac{1}{x-i\epsilon}[/tex]=[tex]\frac{x}{x²+a²}[/tex]+[tex]\frac{ia}{x²+a²}[/tex]= P [tex]\frac{1}{x}[/tex]+i pi [tex]\delta[/tex](x)(adsbygoogle = window.adsbygoogle || []).push({});

P means the principal value, a is possibly infinitesimal (?), i is the imaginary unit

Where does the pi, Dirac delta come from? What principal value?

It is from a quantum field theory book.

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# Homework Help: An identity

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