sentences:  This update maintains the symmetry and positive definiteness of the Hessian matrix.Given a function formula_1, its gradient (formula_2), and positive-definite Hessian matrix formula_3, the Taylor series is
  
  and the Taylor series of the gradient itself (secant equation)
  
  is used to update formula_3.The DFP formula finds a solution that is symmetric, positive-definite and closest to the current approximate value of formula_7