= Pure-radiation constant-pressure heat-capacity singularity
For pure <blackbody radiation>, $P=aT^4/3$ depends only on <temperature>. Constant <pressure> therefore fixes <temperature>, so a usual finite constant-pressure <temperature> derivative is unavailable. In the material gas-radiation limit the <specific-heat ratio> diverges, although all three <stellar adiabatic exponents> approach $4/3$. The radiation index $4/3$ is an adiabatic pressure-volume response, not a finite heat-capacity ratio.
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