Proving recursion relations. BFGS non linear optimization

Click For Summary
SUMMARY

This discussion focuses on proving recursion relations in the context of BFGS non-linear optimization. Key elements include the Hessian approximation (Bk), the search direction (pk), and the step size (α). The relationship yk = ∇f(xk+1) - ∇f(xk) is established, leading to the equation Bk+1(xk+1 - xk) = yk. The discussion emphasizes that Bk is not the exact Hessian but an approximation that improves through rank-1 updates, ultimately converging to the exact Hessian after a finite number of line searches.

PREREQUISITES
  • Understanding of BFGS algorithm in non-linear optimization
  • Familiarity with Hessian matrices and their properties
  • Knowledge of gradient vectors and their role in optimization
  • Basic concepts of rank-1 updates in linear algebra
NEXT STEPS
  • Research the BFGS algorithm and its applications in optimization
  • Study rank-1 update formulas for Hessian approximations
  • Learn about the properties of Hessian matrices in optimization contexts
  • Explore the implications of exact line searches in non-linear optimization
USEFUL FOR

Students and professionals in optimization, particularly those studying or working with non-linear optimization techniques, as well as researchers interested in Hessian approximations and BFGS methods.

sdevoe
Messages
21
Reaction score
0

Homework Statement



Please see attached thumbnail
Here's what I know.
1)Bk is the Hessian
2) sk = \alpha*p
3)pk is the search direction
4) Alpha is the step size

Homework Equations



yk = \nablaf(xk+1) -\nablaf(xk
Bk+1(xk+1-xk) = \nablaf(xk+1) -\nablaf(xk

The Attempt at a Solution


Bk+1(xk+1-xk) = yk

and then somehow from there I have to use the above to prove the Hk+1 equation
 

Attachments

  • Screen shot 2012-04-29 at 9.41.54 PM.png
    Screen shot 2012-04-29 at 9.41.54 PM.png
    9.3 KB · Views: 498
Last edited:
Physics news on Phys.org
sdevoe said:

Homework Statement



Please see attached thumbnail
Here's what I know.
1)Bk is the Hessian
2) sk = \alpha*p
3)pk is the search direction
4) Alpha is the step size

Homework Equations



yk = \nablaf(xk+1) -\nablaf(xk
Bk+1(xk+1-xk) = \nablaf(xk+1) -\nablaf(xk

The Attempt at a Solution


Bk+1(xk+1-xk) = yk

and then somehow from there I have to use the above to prove the Hk+1 equation

You don't know that B_k is the Hessian; you only know that it is a current approximation to the Hessian. In a purely quadratic model with exact line searches, each line search produces a better approximation to the Hessian, so starting from B_0 = Identity matrix (for example), you will have B_k = exact Hessian for some k <= n; that is, in at most n line searches you will get the Hessian---all of this without ever computing a second derivative! Going from B_k to B_{k+1} is a rank-1 update. There are formulas for how to change the inverse after a rank-1 update. I don't remember them, but they are widely available in the linear algebra/nonlinear optimization literature.

RGV
 

Similar threads

Replies
3
Views
2K
  • · Replies 6 ·
Replies
6
Views
3K
Replies
17
Views
2K
  • · Replies 14 ·
Replies
14
Views
2K
Replies
6
Views
2K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K