is sufficient to have at one’s disposal only the effective Lagrangian L ( F , G ) , from where the dependance on the time- and space-derivatives of the field F$_{µν}$ is disregarded [35]. In the limit of vanishing momenta the invariant coefficients Λ$_{1}$$_{,}$$_{2}$$_{,}$$_{3}$$_{,}$$_{4}$ are quadratic functions of k$_{µ}$ expressed in terms of the (momentum-independent) derivatives L$_{F}$ = ∂ L /∂ F , L$_{FF}$ = ∂ $^{2}$L /∂ F $^{2}$, L$_{GG}$ = ∂ $^{2}$L /∂ G $^{2}$, L$_{FG}$ = ∂ $^{2}$L /∂ F ∂ G as follows Λ$_{1}$ | k$_{µ}$ → 0 = ( k 2 3 − k 2 $_{0}$) L$_{F}$ , (17) Λ$_{2}$ | k$_{µ}$ → 0 = − k $^{2}$L$_{F}$ − k 2 $_{⊥}$( B $^{2}$L$_{FF}$ + E $^{2}$L$_{GG}$ + 2 GL$_{FG}$ ) , (18) Λ$_{4}$ | k$_{µ}$ → 0 = − k $^{2}$L$_{F}$ + ( k 2 3 − k 2 $_{0}$)( E $^{2}$L$_{FF}$ + B $^{2}$L$_{GG}$ − 2 GL$_{FG}$ ) , (19) Λ$_{3}$ | k$_{µ}$ → 0 = ( k 2 $_{⊥}$) 1 2 ( k 2 0 − k 2 $_{3}$) 1 2 { L$_{FG}$ ( B 2 + E $^{2}$) − ( L$_{GG}$ + L$_{FF}$ ) G } . (20) We have written these formulae referring to the special frame. The equivalence relations (4) allow to immediately restore their invariant form valid in any frame. Eqs. (17) – (20) are obtained using the definition of the polarization operator components as the second derivatives with respect to vector-potentials components (see, e .g. [3]). Insofar as one is interested in the quantities Λ$_{i}$ | k$_{µ}$ → 0 up to one-loop accuracy one should either take the Heisenberg-Euler expression for L here or pass to the infrared limit in the expressions for Λ$_{i}$ calculated within one-loop approximation in [4]. The two-loop approximation for L is also available [10]. From eqs. (17) – (20) the vanishing of the eigenvalues in the zero-momentum point follows. This property is, in the end, a consequence of the gauge invariance that requires that the effective Lagrangian should depend only on the field strengthes, and not potentials. In the special frame dispersion equations (14) can be represented in the form κ a | k =0 = 0 , a = 1 , 2 , 3 (21) κ $_{a}$( k 2 0 − k 2 $_{3}$, k 2 $_{⊥}$, B $^{2}$, E$^{2}$) = k 2 ⊥ + k 2 3 − k 2 $_{0}$, a = 1 , 2 , 3 (22) 13 III. DISPERSION CURVES