TITLE: What is a Directional Derivative?
# Consider a smooth function of two variables:<br>
  
  <center> z = f(x,y) </center>
  
  the partial derivative of f with respect to y at
  the point (x0,y0) is denoted by fy(x0,y0). Recall
  the definition of 
  <a href="http://omega.albany.edu:8008/mat214dir/partials-dir/define-m2h.html">
  partial derivative</a>.
  Geometrically the number fy(x0,y0) is the slope of the tangent line at the
  point (x0,y0,z0) to a curve on the surface z=f(x,y). This curve is the
  intersection of the plane x=x0 with the surface z=f(x,y). (See figure)

!partial1.gif  

# The partial derivative fy(x0,y0) is a special case of a directional derivative.
  It is the directional derivative in the direction of the unit vector "j".
  If you imagine yourself standing on the surface at the point (x0,y0,z0) 
  and looking in the direction of "j" the slope you see is exactly the
  value of fy(x0,y0). If you turn around and you now find yourself looking in
  the direction of the unit vector "u" say, then the slope you see is
  called the <em>directional derivative in the direction of u</em>.
  Another way to look at it is to imagine the plane in the above figure revolving
  around the axis through the point (x0,y0) (i.e. the line x=x0, y=y0). These 
  planes cut the surface at different curves (dashed curve in the figure). The
  slope of the tangent at the point (x0,y0,z0) to this curve on the surface when
  the plane contains the vector u=(a,b) attached to (x0,y0) is the directional
  derivative in the direction of u.<p>
  
  The algebraic definition of directional derivative is obtained if 
  we think of f as a function of the vector r=(x,y). We have,

>#
                         d             f(r + h j) - f(r)
                       ---- z = Limit  -----------------
                        dy      h -> 0         h

# the partial fy is the rate of change of f when r moves a bit from
  r in the direction of j. i.e. from r to r+hj. If we now let j 
  be an arbitrary unit vector u, we get the formula for the directional
  derivative in the direction of u given by,
  
> #
                                      f(r + h u) - f(r)
                         Duf = Limit  -----------------
                               h -> 0         h

# so the directional derivative of f in the direction of u, "Duf" is
  the rate of change of f when we move from r a little bit in the direction
  of u. i.e. from r to r+hu. in terms of the components we have,
  
> #
                                f(x + h a, y + h b) - f(x, y)
                   Duf = Limit  -----------------------------
                         h -> 0               h

# where we have assumed that u=(a,b). If we now let,
>#
                           F(h) = f(x + h a, y + h b)
# we can see that the derivative of F at h=0 is,
> #

                        d         /       F(h) - F(0)\
                      ---- F(0) = |Limit  -----------|  = Duf
                       dh         \h -> 0      h     /

# but using the 
 <a href="http://omega.albany.edu:8008/mat214dir/chain-rules-dir/examples-m2h.html">
 chain rule</a> we obtain,
 
> Duf = 'diff(f,x)'*a + 'diff(f,y)'*b;

                               /  d   \     /  d   \
                         Duf = |---- f| a + |---- f| b
                               \ dx   /     \ dy   /

# this last formula can be seen as the inner product between the so called
  <em> gradient of f</em> with the direction vector u=(a,b). The gradient
  of f is the vector of partial derivatives. Maple gives you this vector
  with the command,
  
> grad(f(x,y),[x,y]);

                             d             d
                         [ ---- f(x, y), ---- f(x, y) ]
                            dx            dy

# so the above can be stated as a,
  <h4>Theorem</h4>
  The directional derivative of f in the direction of u=(a,b)
  is the projection of the gradient of f onto the direction u.
  Or in symbols,
> Duf = innerprod( grad(f(x,y),[x,y]) , [a,b] );

                         /  d         \     /  d         \
                   Duf = |---- f(x, y)| a + |---- f(x, y)| b
                         \ dx         /     \ dy         /
# <hr>
  The fact that Duf can be written as the innerprod of the gradient
  of f with u has a very important consequence. To see this remember
  the coordinate free definition of inner product. The inner product
  of two vectors, u and v say, is always equal to the length of
  u times the length of v times the cosine of the angle in between u
  and v. If we suppose that the angle between grad(f) and u is t, then,
> #
                             Duf = | grad(f) | cos(t)
# and since cos(t) is always in between 1 and -1 we have that,<br>

    <center> - | grad(f) | < Duf < | grad(f) | </center>
  
  The upper bound for Duf is the length of the gradient of f when 
  t=0 i.e. when u is pointing in the direction of the gradient.
  On the other hand the lower bound for Duf is minus the lenght
  of the gradient of f which is achieved when t=Pi i.e. when u is
  pointing in the direction of minus the gradient of f.<p>
  
  In other words at every point on the smooth surface the
  direction of grad(f) and -grad(f) at that point show where we need to
  look to find the directions of steepest ascend  and steepest
  descent on the surface at that point. This fact can be exploited 
  to find fast ways to climb surfaces (or functions of several variables).
  
>