The allowed set of values of the constants in an eigenvalue equation. In an eigenvalue equation the left-hand side consists of an operator and an eigenfunction, upon which the operator operates; the right-hand side of the equation consists of the product of the same eigenfunction and a constant. Eigenvalue equations occur in quantum mechanics, with the eigenvalues being the values of the quantized quantities. In particular, the Schrödinger equation is an eigenvalue equation with the allowed quantized energy levels being the eigenvalues of this equation.