Vector quasi_newton(Vector& x, F f, Grad grad_f, Step step, Update update, Iter& iter)
{
typedef typename mtl::Collection<Vector>::value_type value_type;
Vector d, y, x_k, s;
Matrix H(size(x), size(x));
H= 1;
for (; !iter.finished(two_norm(grad_f(x))); ++iter) {
d= H * -grad_f(x); // std::cout << "d is " << d << '\n';
value_type alpha= step(x, d, f, grad_f); assert(alpha == alpha);
x_k= x + alpha * d; // std::cout << "x_k is " << x_k << '\n';
s= alpha * d; // std::cout << "alpha is " << alpha << '\n';
y= grad_f(x_k) - grad_f(x);
update(H, y, s);
x= x_k;
}
return x;
}
