g"
 ALT="$u_1$"> and <IMG
 WIDTH="15" HEIGHT="10" ALIGN="MIDDLE" BORDER="0"
 SRC="img52.png"
 ALT="$u_2$"> are known  as <EM>internal</EM> variables
for   the   element  function.   They  are obtained   as  <EM>linear
combinations</EM>  of the elemental  variables.   The important feature as
far as an optimization procedure is concerned  is that  each nonlinear
function  involves  as  few internal  variables  as possible,  as this
allows  for   compact    storage   and  more  efficient     derivative
approx