= Covariance of a selected particle momentum sum
{title2=$P_A^\mu=\sum_{i\in A}p_i^\mu$}
A fixed set of four-momenta at fixed events sums to a four-vector, whether or not the set is the whole physical system. Its Minkowski norm is invariant. Reapplying a frame-dependent particle-selection rule or sampling different events need not give the transformed same sum. Physical conservation across different hypersurfaces is a separate issue involving interactions and boundary flux.
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