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## Main Question or Discussion Point

**Unknown "well-known" equation in Sakurai**

On p391 in Modern Quantum Mechanics is stated: Now we use the well-known equation:

[tex]\frac{1}{E - H_0 -i \epsilon} = Pr. \left( \frac{1}{E-H_0} \right) + i \pi \delta \left(E-H_0 \right)[/tex]

Frankly I've never heard of the equation or the notation Pr. My first thought is that the Pr. stands for Cachy principal value integral, but that's denoted [tex]P \int[/tex] or P.v. So I'm lost here...