Editing Maximum likelihood estimation (section)

==== [[Davidon–Fletcher–Powell formula]] ====
DFP formula finds a solution that is symmetric, positive-definite and closest to the current approximate value of second-order derivative:
<math display="block">\mathbf{H}_{k+1} =
  \left(I - \gamma_k y_k s_k^\mathsf{T}\right) \mathbf{H}_k \left(I - \gamma_k s_k y_k^\mathsf{T}\right) + \gamma_k y_k y_k^\mathsf{T},
</math>

where

<math display="block">y_k = \nabla\ell(x_k + s_k) - \nabla\ell(x_k),</math>
<math display="block">\gamma_k = \frac{1}{y_k^T s_k},</math>
<math display="block">s_k = x_{k+1} - x_k.</math>