Fermion four-current density

In the relativistic version of DFT, the ground state energy of the system is a unique functional of the fermion four-current density, / (/-) ... 

In this equation, A, is a given external four-potential and J (x) is the four-current density. As the treatment is for stationary states, the external four-potential and the fermion four-current are independent of time. 

The various bilinear fermionic terms in the interaction Lagrangian are four-currents that interact with the gauge bosons. The Lagrange density... 

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