Einstein–dilaton–four-Maxwell holographic anisotropic models

In recent studies on holographic QCD, the consideration of five-dimensional Einstein–dilaton–Maxwell models has played a crucial role. Typically, one Maxwell field is associated with the chemical potential, while additional Maxwell fields are used to describe the anisotropy of the model. A more general scenario involves up to four Maxwell fields. The second field represents spatial longitudinal–transverse anisotropy, while the third and fourth fields describe anisotropy induced by an external magnetic field. We consider an ansatz for the metric characterized by four functions at zero temperature and five functions at nonzero temperature. The Maxwell field related to the chemical potential is treated with the electric ansatz, as is customary, whereas the remaining three Maxwell fields are treated with a magnetic ansatz. We demonstrate that in the fully anisotropic diagonal case, only six out of the seven equations are independent. One of the matter equations (either the dilaton or the vector potential equation) follows from the Einstein equations and the remaining matter equation. This redundancy arises due to the Bianchi identity for the Einstein tensor and the specific form of the energy–momentum tensor in the model. A procedure for solving this system of six equations is provided. This method generalizes previously studied cases involving up to three Maxwell fields. In the solution with three magnetic fields case, our analysis shows that the dilaton equation is a consequence of the five Einstein equations and the equation for the vector potential.