.  Then one can consider
the nonlinear element function
to be of the <EM>structural</EM> form <!-- MATH
 $f_j
(v_1,\cdots,v_{n_j})$
 -->
<IMG
 WIDTH="99" HEIGHT="18" ALIGN="MIDDLE" BORDER="0"
 SRC="img34.png"
 ALT="$f_j
(v_1,\cdots,v_{n_j})$">, where   we assign  <!-- MATH
 $v_1  = \bar{x}_{j1},\cdots,
v_{n_j} =  \bar{x}_{jn_{j}}$
 -->
<IMG
 WIDTH="164" HEIGHT="16" ALIGN="MIDDLE" BORDER="0"
 SRC="img35.png"
 ALT="$v_1 = \bar{x}_{j1},\cdots,
v_{n_j} = \bar{x}_{jn_{j}}$">.  The <EM>elemental</EM> variables
fo