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.
Projection (linear algebra)
When the vector space formula_38 has an inner product and is complete (is a Hilbert space) the concept of orthogonality can be used. An orthogonal projection is a projection for which the range formula_41 and the null space formula_4 are orthogonal subspaces. Thus, for every formula_76 and formula_77 in formula_38, formula_79. Equivalently: