v 11.11

ρ_{f} = \frac{A_{21}}{B_{21}} \frac{1}{\exp (\frac{h f}{k_{B} T}) - 1}

${\rho_f=\frac{A_{21}}{B_{21}} \frac{1}{\exp \big(\frac{hf}{k_B T} \big)-1}}$

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