Appendix 1
The matrices appearing in Eq. (20) are as follows:
$$ \begin{aligned} {\mathbf{D}}_{1} & = \left[ {\begin{array}{*{20}l} {\bar{A}\left( {1,1} \right)} \hfill & {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {1,7} \right)} \hfill \\ {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {2,2} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {2,7} \right)} \hfill \\ {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {3,7} \right)} \hfill \\ {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {4,4} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {4,7} \right)} \hfill \\ {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {5,5} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {5,7} \right)} \hfill \\ {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {6,6} \right)} \hfill & {\bar{A}\left( {6,7} \right)} \hfill \\ \end{array} } \right], \\ {\mathbf{N}}_{{{\text{s}}1}} & = - \left[ {\begin{array}{*{20}l} {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {1,7} \right)} \hfill \\ {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {2,2} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {2,7} \right)} \hfill \\ {\bar{A}\left( {3,3} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {3,7} \right)} \hfill \\ {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {4,4} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {4,7} \right)} \hfill \\ {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {5,5} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {5,7} \right)} \hfill \\ {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {6,6} \right)} \hfill & {\bar{A}\left( {6,7} \right)} \hfill \\ \end{array} } \right], \\ {\mathbf{N}}_{{{\text{s}}2}} & = - \left[ {\begin{array}{*{20}l} {\bar{A}\left( {1,1} \right)} \hfill & {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {1,7} \right)} \hfill \\ {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {2,7} \right)} \hfill \\ {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {3,3} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {3,7} \right)} \hfill \\ {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {4,4} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {4,7} \right)} \hfill \\ {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {5,5} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {5,7} \right)} \hfill \\ {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {6,6} \right)} \hfill & {\bar{A}\left( {6,7} \right)} \hfill \\ \end{array} } \right], \\ \end{aligned} $$
$$ {\mathbf{N}}_{{{\text{s}}3}} = - \left[ {\begin{array}{*{20}l} {\bar{A}\left( {1,1} \right)} \hfill & {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {1,7} \right)} \hfill \\ {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {2,2} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {2,7} \right)} \hfill \\ {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {3,3} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {3,7} \right)} \hfill \\ {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {4,7} \right)} \hfill \\ {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {5,5} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {5,7} \right)} \hfill \\ {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {6,6} \right)} \hfill & {\bar{A}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\mathbf{s}}4}} = - \left[ {\begin{array}{*{20}l} {\bar{A}\left( {1,1} \right)} \hfill & {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {1,7} \right)} \hfill \\ {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {2,2} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {2,7} \right)} \hfill \\ {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {3,3} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {3,7} \right)} \hfill \\ {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {4,4} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {4,7} \right)} \hfill \\ {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {5,7} \right)} \hfill \\ {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {6,6} \right)} \hfill & {\bar{A}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{s}}5}} = - \left[ {\begin{array}{*{20}l} {\bar{A}\left( {1,1} \right)} \hfill & {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {1,7} \right)} \hfill \\ {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {2,2} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {2,7} \right)} \hfill \\ {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {3,3} \right)} \hfill & {\bar{A}\left( {3,7} \right)} \hfill \\ {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {4,4} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {4,7} \right)} \hfill \\ {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {5,5} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {5,7} \right)} \hfill \\ {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{s}}6}} = - \left[ {\begin{array}{*{20}l} {\bar{A}\left( {1,1} \right)} \hfill & {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {1,3} \right)} \hfill \\ {\bar{A}\left( {1,2} \right)} \hfill & {\bar{A}\left( {2,2} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill \\ {\bar{A}\left( {1,3} \right)} \hfill & {\bar{A}\left( {2,3} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill & {\bar{A}\left( {3,3} \right)} \hfill \\ {\bar{A}\left( {1,4} \right)} \hfill & {\bar{A}\left( {2,4} \right)} \hfill & {\bar{A}\left( {4,4} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {3,4} \right)} \hfill \\ {\bar{A}\left( {1,5} \right)} \hfill & {\bar{A}\left( {2,5} \right)} \hfill & {\bar{A}\left( {4,5} \right)} \hfill & {\bar{A}\left( {5,5} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {3,5} \right)} \hfill \\ {\bar{A}\left( {1,6} \right)} \hfill & {\bar{A}\left( {2,6} \right)} \hfill & {\bar{A}\left( {4,6} \right)} \hfill & {\bar{A}\left( {5,6} \right)} \hfill & {\bar{A}\left( {6,6} \right)} \hfill & {\bar{A}\left( {3,6} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{D}}_{2} = \left[ {\begin{array}{*{20}l} {\bar{B}\left( {1,1} \right)} \hfill & {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {1,7} \right)} \hfill \\ {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {2,2} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {2,7} \right)} \hfill \\ {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {3,7} \right)} \hfill \\ {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {4,4} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {4,7} \right)} \hfill \\ {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {5,5} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {5,7} \right)} \hfill \\ {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {6,6} \right)} \hfill & {\bar{B}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{r}}1}} = - \left[ {\begin{array}{*{20}l} {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {1,7} \right)} \hfill \\ {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {2,2} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {2,7} \right)} \hfill \\ {\bar{B}\left( {3,3} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {3,7} \right)} \hfill \\ {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {4,4} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {4,7} \right)} \hfill \\ {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {5,5} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {5,7} \right)} \hfill \\ {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {6,6} \right)} \hfill & {\bar{B}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{r}}2}} = - \left[ {\begin{array}{*{20}l} {\bar{B}\left( {1,1} \right)} \hfill & {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {1,7} \right)} \hfill \\ {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {2,7} \right)} \hfill \\ {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {3,3} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {3,7} \right)} \hfill \\ {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {4,4} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {4,7} \right)} \hfill \\ {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {5,5} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {5,7} \right)} \hfill \\ {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {6,6} \right)} \hfill & {\bar{B}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{r}}3}} = - \left[ {\begin{array}{*{20}l} {\bar{B}\left( {1,1} \right)} \hfill & {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {1,7} \right)} \hfill \\ {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {2,2} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {2,7} \right)} \hfill \\ {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {3,3} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {3,7} \right)} \hfill \\ {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {4,7} \right)} \hfill \\ {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {5,5} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {5,7} \right)} \hfill \\ {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {6,6} \right)} \hfill & {\bar{B}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{r}}4}} = - \left[ {\begin{array}{*{20}l} {\bar{B}\left( {1,1} \right)} \hfill & {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {1,7} \right)} \hfill \\ {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {2,2} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {2,7} \right)} \hfill \\ {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {3,3} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {3,7} \right)} \hfill \\ {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {4,4} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {4,7} \right)} \hfill \\ {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {5,7} \right)} \hfill \\ {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {6,6} \right)} \hfill & {\bar{B}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{r}}5}} = - \left[ {\begin{array}{*{20}l} {\bar{B}\left( {1,1} \right)} \hfill & {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {1,7} \right)} \hfill \\ {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {2,2} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {2,7} \right)} \hfill \\ {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {3,3} \right)} \hfill & {\bar{B}\left( {3,7} \right)} \hfill \\ {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {4,4} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {4,7} \right)} \hfill \\ {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {5,5} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {5,7} \right)} \hfill \\ {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {6,7} \right)} \hfill \\ \end{array} } \right], $$
$$ {\mathbf{N}}_{{{\text{r}}6}} = - \left[ {\begin{array}{*{20}l} {\bar{B}\left( {1,1} \right)} \hfill & {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {1,3} \right)} \hfill \\ {\bar{B}\left( {1,2} \right)} \hfill & {\bar{B}\left( {2,2} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill \\ {\bar{B}\left( {1,3} \right)} \hfill & {\bar{B}\left( {2,3} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill & {\bar{B}\left( {3,3} \right)} \hfill \\ {\bar{B}\left( {1,4} \right)} \hfill & {\bar{B}\left( {2,4} \right)} \hfill & {\bar{B}\left( {4,4} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {3,4} \right)} \hfill \\ {\bar{B}\left( {1,5} \right)} \hfill & {\bar{B}\left( {2,5} \right)} \hfill & {\bar{B}\left( {4,5} \right)} \hfill & {\bar{B}\left( {5,5} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {3,5} \right)} \hfill \\ {\bar{B}\left( {1,6} \right)} \hfill & {\bar{B}\left( {2,6} \right)} \hfill & {\bar{B}\left( {4,6} \right)} \hfill & {\bar{B}\left( {5,6} \right)} \hfill & {\bar{B}\left( {6,6} \right)} \hfill & {\bar{B}\left( {3,6} \right)} \hfill \\ \end{array} } \right], $$
(47)
Appendix 2
Figure 2b illustrates that the degenerated HTFE consists of four inter-elements. The explicit forms of the shape function matrix \( {\mathbf{N}}_{i} \) appearing in Eq. (29) for the \( i \)th (\( i \) = 1, 2, 3, 4) inter-element are given by
$$ \begin{aligned} & {\mathbf{N}}_{1} = \left[ {\begin{array}{*{20}c} {\hat{n}_{1} {\mathbf{I}}} & {\hat{n}_{2} {\mathbf{I}}} & {\mathbf{O}} & {\mathbf{O}} \\ \end{array} } \right],{\mathbf{N}}_{2} = \left[ {\begin{array}{*{20}c} {\mathbf{O}} & {\hat{n}_{1} {\mathbf{I}}} & {\hat{n}_{2} {\mathbf{I}}} & {\mathbf{O}} \\ \end{array} } \right], \\ & {\mathbf{N}}_{3} = \left[ {\begin{array}{*{20}c} {\mathbf{O}} & {\mathbf{O}} & {\hat{n}_{2} {\mathbf{I}}} & {\hat{n}_{1} {\mathbf{I}}} \\ \end{array} } \right]\;{\text{and}}\;{\mathbf{N}}_{4} = \left[ {\begin{array}{*{20}c} {\hat{n}_{1} {\mathbf{I}}} & {\mathbf{O}} & {\mathbf{O}} & {\hat{n}_{2} {\mathbf{I}}} \\ \end{array} } \right] \\ \end{aligned} $$
(48)
in which \( {\mathbf{I}} \) is a (\( 7 \times 7 \)) identity matrix while \( \hat{n}_{1} \) and \( \hat{n}_{2} \) are the shape functions associated with the nodes of the inter-element corresponding to the values of the natural coordinate as \( \zeta = - 1 \) and \( \zeta = 1 \), respectively. These are given by
$$ \hat{n}_{1} = \frac{1}{2}\left( {1 - \zeta } \right)\,{\text{and}}\,\hat{n}_{2} = \frac{1}{2}\left( {1 + \zeta } \right) $$
(49)