x = (x,y,z,r,s,t)$ and $d^6x = dx\,dy\,dz\,dr\,ds\,dt$. $H_i(\vec x)$ denotes the full numerator, including regular factors of the Jacobians due to the Feynman parametrization and substitutions, and singular as well as higher powers of the dimensional regulator $\epsilon$, and $N(\vec x)$ the final denominator, $$\begin{aligned}
N(\vec x) & = & 1 + \rho_s xzr \Big\{ xz + (1-x) [zs+(1-s)t] \Big\}
\nonumber \\
& & \quad - \rho_t x \Big\{ z(1-y-r) + (y-z)[z+(1-x)(1-s)(t-z)] \Big\}
\nonumber \\
& & \quad + \rho_u xzr \Big\{ xz + (1-x) [zs+(1-s)y] \Big\} \nonumber \\
& & \quad - \rho_H \Big\{ [xz+(1-x)y][1-xz-(1-x)y] - x(1-x)s(y-z)^2
\Big\} \, ,\end{aligned}$$ where we define $\rho_s = \hat s/m_t^2 = Q^2/m_t^2$, $\rho_t = (\hat
t-M_H^2)/m_t^2$, $\rho_u = (\hat u-M_H^2)/m_t^2$ and $\rho_H =
M_H^2/m_t^2$. The singular powers in $\epsilon$ of $H_i(\vec x)$ arise from powers of $k^2$ and $q^2$ in the numerators of the final integrations of the loop momenta $k$ and $q$. It is important that the final denominator develops the form of $1+O(1/m_t^2)$ to ensure that no further ultraviolet nor infrared singularities arise from this part of the integrand.

The integral for $\Delta F_i$ of Eq. (\[eq:fi39\]) is singular for $s\to 0$. To separate this singularity from the integral, we 