contributions with respect to the index $\mu $ allows three of the eight coefficient functions, $\lambda $, to be immediately determined from the WTI (\[WTI\]) in terms of the self-energy functions $A$ and $B$ given in (\[2.2\]) and (\[2.3\]) respectively. The longitudinal coefficient functions are thus given by $$\begin{aligned}
\frac{P\cdot q}{\eta }\lambda ^L_1&=&B(P^2_-)-B(P^2_+)\nonumber \\
\frac{P\cdot q}{\eta ^2}\lambda ^L_2&=&A(P^2_-)-A(P^2_+)\label{2.9} \\
\lambda ^L_3&=&\frac{1}{2}\left[ A(P^2_-)+A(P^2_+)\right]  .\nonumber\end{aligned}$$ The additional constraint provided by the WI (\[WI\]) obtains the soft-photon limits: $$\begin{aligned}
\left. \lambda ^L_1\right| _{q=0}=\left. \lambda ^T_1\right|
_{q=0}&=&-2\eta B'(P^2)\nonumber \\
\left. \lambda ^L_2\right| _{q=0}=\left. \lambda ^T_2\right|
_{q=0}&=&-2\eta ^2A'(P^2)\label{2.10}\\
\left. \lambda ^L_3\right| _{q=0}=\left. \lambda ^T_3\right|
_{q=0}&=&A(P^2),\nonumber\end{aligned}$$ where the prime denotes differentiation with respect to the argument.

The five unknown transverse coefficients can be isolated using (\[2.6\]) and (\[2.8\]), and the orthogonality of the Dirac matrices. The closed set of equations so obtained can be written in matrix form as $$\begin{aligned}
b_i&=&{\cal N}_{ij}(P^2,C_{Pq}^2;q^2)\lambda ^T_j(P^2,C_{Pq}^2;q^2)\nonumber \\
& & \; \; \; \; \; \; + \int _{0}^{\infty }dK K^3\int
_{-1}^{1}dC_{Kq}{\cal M}_{ij}(P^2,C_{Pq}^2,K^2,C_{Kq}^2;q^2)\lambda
^T_j(K^2,C_{Kq}^2;q^2),\label{2.16}\end{aligned}$$ where ${\cal M}$ and ${\cal N}$($b$ and $\lambda ^T$) are matrices(vectors) in the five-component space of the coefficient functions and $C_{Kq}$ is the direction cosine between $K$ and $q$. The steps required to reach the result 