C_F T_F {N_{\!f}}\right] a^2 \nonumber \\
&& +~ \frac{1}{243} \left[ \left( 7776 \zeta(3) + 4264 \right) C_A C_F^2
- \left( 2592 \zeta(3) + 10460 \right) C_A^2 C_F \right. \nonumber \\
&& \left. ~~~~~~~~~~+~ \left( 5184 \zeta(3) + 3128 \right) C_A C_F T_F {N_{\!f}}- \left( 5184 \zeta(3) - 280 \right) C_F^3 \right. \nonumber \\
&& \left. ~~~~~~~~~~-~ \left( 5184 \zeta(3) - 3412 \right) C_F^2 T_F {N_{\!f}}+ 448 C_F T_F^2 {N_{\!f}}^2 \right] a^3 ~+~ O(a^4) ~, \nonumber \\
\gamma^{W_3}_{33}(a) &=& O(a^4)
\label{anom3}\end{aligned}$$ where $\zeta(z)$ is the Riemann zeta function, $a$ $=$ $g^2/(16\pi^2)$ and ${N_{\!f}}$ is the number of massless quarks. The group Casimirs are defined by $$T^a T^a ~=~ C_F ~~,~~ \mbox{Tr} \left( T^a T^b \right) ~=~ 
T_F \delta^{ab} ~~,~~ f^{acd} f^{bcd} ~=~ C_A \delta^{ab}$$ where $T^a$ are the Lie group generators with structure functions $f^{abc}$ and $1$ $\leq$ $a$ $\leq$ ${N_{\!A}}$ where ${N_{\!A}}$ is the dimension of the adjoint representation.

We turn now to the set-up for the particular Green’s function we are interested in which is $\left\langle \psi(p) {\cal O}^i_{\mu_1 \ldots \mu_{n_i}} (-p-q) 
\bar{\psi}(q) \right\rangle$ and is illustrated in Figure $1$. The independent external momenta we use are $p$ and $q$ and are the momenta flowing into the external quark 