function grad = extrosenfngrad(x)
% usage: y = extrosenfngrad(x)
% description: Returns value of function of function of
% Exercise 9.1 of Nocedal and Wright's text, Numerical
% Optimization.  This is the extended Rosenbrock
% function. It is *assumed* that length(x) is even!
% Solution is x* = (1,1,...,1), minimum value 0.

% no bulletproofing
% for adapting this problem, change c here and in
% original and hessian functions.

c = 10;
m = length(x);
grad = zeros(size(x));
grad(1:2:m-1) = 2*(x(1:2:m-1)-1) - 4*c*(x(2:2:m)-x(1:2:m-1).^2).*x(1:2:m-1);
grad(2:2:m) = 2*c*(x(2:2:m)-x(1:2:m-1).^2);