definition of gravitational strength can be expressed as, $$\begin{aligned}
\mathbb{F}_g = ( - \partial_0 a_0 + \nabla \cdot \textbf{a} ) + \emph{i} ( \partial_0 \textbf{a} + \nabla a_0 ) + \nabla \times \textbf{a} ~,\end{aligned}$$ where $\textbf{a} = \Sigma a_k \emph{\textbf{i}}_k$; $a_0 = \varphi / v_0$, with $\varphi$ being the scalar potential of gravitational field.

For the sake of convenience the paper adopts the gauge condition, $f_0 = - \partial_0 a_0 + \nabla \cdot \textbf{a} = 0$. Further the above can be written as $\textbf{f} = \emph{i} \textbf{g} / v_0 + \textbf{b}$, with $\textbf{f} = \Sigma f_k \emph{\textbf{i}}_k$. One component of gravitational strength is the gravitational acceleration, $\textbf{g} / v_0 = \partial_0 \textbf{a} + \nabla a_0$. The other is $\textbf{b} = \nabla \times \textbf{a}$, which is similar to the magnetic flux density.

In the definition of gravitational source Eq.(3), the gravitational source is, $\mathbb{S}_g = \emph{i} s_0 + \textbf{s}$. Substituting the gravitational source $\mathbb{S}_g$ and gravitational strength $\mathbb{F}_g$ into the definition of gravitational source can express the definition as, $$- \mu_g ( \emph{i} s_0 + \textbf{s} ) = ( \emph{i} \partial_0 + \nabla )^* \circ ( \emph{i} \textbf{g} / v_0 + \textbf{b} ) ~,$$ where the gravitational coefficient is $\mu_g = 