In heterotic string theory, the Strominger's equations are the set of equations that are necessary and sufficient conditions for spacetime supersymmetry. It is derived by requiring the 4-dimensional spacetime to be maximally symmetric, and adding a warp factor on the internal 6-dimensional manifold. Consider a metric \omega on the real 6-dimensional internal manifold Y and a Hermitian metric h on a vector bundle V. The equations are: where R^{-} is the Hull-curvature two-form of \omega, F is the curvature of h, and \Omega is the holomorphic n-form; F is also known in the physics literature as the Yang-Mills field strength. Li and Yau showed that the second condition is equivalent to \omega being conformally balanced, i.e.,. These equations imply the usual field equations, and thus are the only equations to be solved. However, there are topological obstructions in obtaining the solutions to the equations; In case V is the tangent bundle T_Y and \omega is Kähler, we can obtain a solution of these equations by taking the Calabi–Yau metric on Y and T_Y. Once the solutions for the Strominger's equations are obtained, the warp factor \Delta, dilaton \phi and the background flux H, are determined by