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In algebraic geometry, the Kempf–Ness theorem, introduced by Kempf and Ness (1979), gives a criterion for the stability of a vector in a representation of a complex reductive group. If the complex vector space is given a norm that is invariant under a maximal compact subgroup of the reductive group, then the Kempf–Ness theorem states that a vector is stable if and only if the norm attains a minimum value on the orbit of the vector.

References

Kempf, George; Ness, Linda (1979), "The length of vectors in representation spaces", Algebraic geometry (Proc. Summer Meeting, Univ. Copenhagen, Copenhagen, 1978), Lecture Notes in Math. 732, Berlin, New York: Springer-Verlag, pp. 233–243, doi:10.1007/BFb0066647, MR 555701