content_image/352413.jpg content_image/352414.jpg content_image/352415.jpg content_image/352416.jpg
frame. Referring to the fact that in the special frame the momentum-containing invariants become 

<img src='content_image/352413.jpg'>

with the two-dimensional vector k$_{⊥}$ being the momentum projection onto the plane orthogo- nal to the common direction 3 of the electric and magnetic field we shall use the equivalence relations 

throughout the paper. 

<img src='content_image/352414.jpg'>

Polarization operator Π$_{µν}$( x, y ) is responsible for small perturbations above the constant- field background. It follows from the translation- Lorentz-, gauge-, PT- and charge- invariance [4, 5] that its Fourier transform can be presented in a diagonal form 

<img src='content_image/352415.jpg'>

where ♭ ( a ) τ are its eigenvectors 

<img src='content_image/352416.jpg'>

while the eigenvalues κ a are scalar functions of F , G , kF $^{2}$k and k ˜ F $^{2}$k. 

The fourth eigenvector is trivial, ♭ (4) µ = k$_{µ}$ , so the fourth eigenvalue vanishes, κ 4 = 0 , as a consequence of the 4-transverseness of the polarization operator, Π$_{µτ}$ k$_{τ}$ = 0. All eigenvectors are mutually orthogonal, ♭ ( a ) µ ♭ ( b ) µ ∼ δ$_{ab}$ , this means that the first three ones are 4-transversal, ♭ ( a ) µ k$_{µ}$ = 0. 

In the special case, where the second field invariant disappears, G = 0 , the three mean- ingful eigenvectors ♭ (1 , 2 , 3) µ are known [4, 5, 14] in the universal final form: 

