= Particle BRST charge and Klein–Gordon constraint
{title2=$Q=c(p^2+\mu^2)/2$}
The <FP ghost> square vanishes, so this <BRST charge> is nilpotent. On a ghost-number-zero wavefunction, <BRST cohomology> implements the <Klein-Gordon equation>. Closure of an unrestricted ghost-extended wavefunction does not independently constrain every component: a pure <FP ghost> component can lie in the kernel without satisfying the same scalar equation. The <FP ghost> sector is part of the physical prescription.
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