/* --------------------------------- */
/* save as x_fxyz.h                     */
/* --------------------------------- */
double fxyz_x(
double (*P_f)(double x, double y, double z),
double   h,
pt3d  p
)
{
double tplsh;
double tmnsh;

 tplsh = ((*P_f)(p.x+h,p.y,p.z));
 tmnsh = ((*P_f)(p.x-h,p.y,p.z));

 return(( tplsh-tmnsh)/(2.*h) );
}
/* --------------------------------- */
double fxyz_y(
double (*P_f)(double x, double y, double z),
double   h,
pt3d  p
)
{
double tplsh;
double tmnsh;

 tplsh = ((*P_f)(p.x,p.y+h,p.z));
 tmnsh = ((*P_f)(p.x,p.y-h,p.z));

 return(( tplsh-tmnsh)/(2.*h) );
}
/* --------------------------------- */
double fxyz_z(
double (*P_f)(double x, double y, double z),
double   h,
pt3d  p
)
{
double tplsh;
double tmnsh;

 tplsh = ((*P_f)(p.x,p.y,p.z+h));
 tmnsh = ((*P_f)(p.x,p.y,p.z-h));

 return(( tplsh-tmnsh)/(2.*h) );
}
/* --------------------------------- 

         First partial derivatives.

 f_x  =  [f(x+h,y,z)-f(x-h,y,z)]/2h

 f_y  =  [f(x,y+h,z)-f(x,y-h,z)]/2h

 f_z  =  [f(x,y,z+h)-f(x,y,z-h)]/2h
   --------------------------------- */
/* --------------------------------- */