Conformal field theory
The Hilbert space of physical states is a unitary module of the Virasoro algebra corresponding to a fixed value of "c". Stability requires that the energy spectrum of the Hamiltonian be nonnegative. The modules of interest are the highest weight modules of the Virasoro algebra.
Hilbert symbol
The Hilbert symbol can also be used to denote the central simple algebra over "K" with basis 1,"i","j","k" and multiplication rules formula_4, formula_5, formula_6. In this case the algebra represents an element of order 2 in the Brauer group of "K", which is identified with -1 if it is a division algebra and +1 if it is isomorphic to the algebra of 2 by 2 matrices.