A functorial approach to the stability of vector bundles

On a normal projective variety the locus of $\mu$-stable bundles that remain $\mu$-stable on all Galois covers prime to the characteristic is open in the moduli space of Gieseker semi-stable sheaves. On a smooth projective curve of genus at least 2 this locus is big in the moduli space of stable bundles, i.e., its complement has codimension at least 2.