# Derivation Heaviside Theta function

 07.01.2018, 13:50 diracisapro Auf diesen Beitrag antworten » Derivation Heaviside Theta function Meine Frage: Hey everyone, I try to understand the derivation of the Lippmann Schwinger equation. In one particular step there is written in our script: $\int \frac{d \omega}{2 \pi} \frac{\exp{-i \omega (t-t')}}{\hbar - \epsilon_{k} + i\eta} = -i \theta (t-t') \exp{-(\frac{i}{\hbar})\epsilon_{k} (t-t')}$ I really dont understand how to get the $theta$ function here. Maybe someone here can help me. Meine Ideen: No ideas yet. Thankful for everything. Cheers
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