= Electron removal spectral function
{title2=$f(\epsilon)A(\mathbf k,\epsilon)$}
The <Electron> single-particle <spectral function> $A(\mathbf k,\epsilon)=-\operatorname{Im}G^R(\mathbf k,\epsilon)/\pi$, in a convention normalized to unit energy integral per orbital and spin, describes addition and removal energies. For the fermionic retarded <Green function>, $G^R(\mathbf k,t)=-i\Theta(t)\langle\{c_{\mathbf k}(t),c_{\mathbf k}^\dagger(0)\}\rangle$, energy is referenced to the <chemical potential>. The occupied removal spectrum is $f(\epsilon)A(\mathbf k,\epsilon)$, with <Fermi-Dirac distribution> $f$. In the noninteracting limit, $A$ is a delta peak at the band energy; interactions can shift, broaden, and redistribute its weight.
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