Given a function f of n variables , we define the partial derivative relative to variable , written as , to be the derivative of f with respect to treating all variables except as constant.
The answer is: .
Let x denote the vector . With this notation, , , etc. The gradient of f at x, written , is the vector . The gradient vector gives the direction of steepest ascent of the function f at point x. The gradient acts like the derivative in that small changes around a given point can be estimated using the gradient.
where denotes the vector of changes.
In this case, and . Since and , we get
Second partials are obtained from f(x) by taking the derivative relative to (this yields the first partial ) and then by taking the derivative of relative to . So we can compute , and so on. These values are arranged into the Hessian matrix
The Hessian matrix is a symmetric matrix, that is .
Example 1.1.1 (continued): Find the Hessian matrix of
The answer is