with integration boundaries from 0 to 1. To symmetrize the $n$-dimensional $k$-integration, we have to perform the shift $$\begin{aligned}
k & \to & k - Q_1 \, , \nonumber \\
Q_1 & = & (1-x)q + xzq_1 + xzr q_2 - [(1-x)y+xz] p_1 \, ,\end{aligned}$$ in both the numerator and denominator. The residual (properly normalized) denominator after the $k$-integration is treated as a propagator for the second loop integration over $q$. We attribute additional Feynman parameters $x_5, x_6$ to this residual propagator and the next one \[$(q-q_1)^2$\] and $1-x_5-x_6$ for the last one ($q^2$) in Eq. (\[eq:mat39\]). Performing the substitution[^6] $$x_5 = s\, , \quad x_6 = (1-s)t\, ,$$ we again arrive at integrals over $s,t$ from 0 to 1. This latter parametrization requires the shift $$\begin{aligned}
q & \to & q - Q_2 \, , \nonumber \\
Q_2 & = & - [zs + (1-s)t] q_1 - zrs q_2 - (y-z)s p_1\end{aligned}$$ in the numerator and denominator to be able to perform the loop integration over $q$ symmetrically. After projecting on the two form factors, we finally arrive at integrals of the type $$\Delta F_i = \frac{\alpha_s}{\pi}~\Gamma(1+2\epsilon)
\left(\frac{4\pi\mu_0^2}{m_t^2}\right)^{2\epsilon} \int_0^1 d^6 x~
\frac{x^\epsilon(1-x)^\epsilon s^{-1-\epsilon} H_i(\vec
x)}{N^{3+2\epsilon}(\vec x)}
\label{eq:fi39}$$ with $\vec 