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.
Projective Hilbert space
In mathematics and the foundations of quantum mechanics, the projective Hilbert space formula_1 of a complex Hilbert space formula_2 is the set of equivalence classes of vectors formula_3 in formula_2, with formula_5, for the relation formula_6 given by