Additional Methods - Nonlinear Programming - Lecture Slides, Slides of Computer Science

These are the Lecture Slides of Nonlinear Programming which includes Convex Cost, Linear Constraints, Duality Theorem, Linear Programming Duality, Quadratic Programming Duality, Linear Inequality, Constrained Problem, Minimize, Feasible etc.Key important points are: Additional Methods, Least-Squares Problems, Incremental, Conjugate Direction Methods, Conjugate Gradient Method, Quasi-Newton Methods, Coordinate Descent Methods, Recall the Least-Squares Problem, Minimize, Steepest Descent Method

Typology: Slides

2012/2013

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NONLINEAR PROGRAMMING
LECTURE 7: ADDITIONAL METHODS
LECTURE OUTLINE
Least-Squares Problems and Incremental Gra-
dient Methods
Conjugate Direction Methods
The Conjugate Gradient Method
Quasi-Newton Methods
Coordinate Descent Methods
Recall the least-squares problem:
minimize f(x)=
1
2
2
g(x)2 = 1
m
i=1 gi(x)2
subject to x ∈
n
,
where g =(g1,...,g
m), gi : n →
ri
.
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NONLINEAR PROGRAMMING

LECTURE 7: ADDITIONAL METHODS

LECTURE OUTLINE

  • Least-Squares Problems and Incremental Gra- dient Methods
  • Conjugate Direction Methods
  • The Conjugate Gradient Method
  • Quasi-Newton Methods
  • Coordinate Descent Methods
  • Recall the least-squares problem:

minimize f (x) = 12 ‖g(x)‖^2 = 21

m

i=

‖gi(x)‖^2

subject to x ∈ n^ ,

where g = (g 1 ,... , gm), gi : n^ → ri†.

INCREMENTAL GRADIENT METHODS

  • Steepest descent method

xk+1^ = xk^ −αk∇f (xk) = xk^ −αk

m

i=

∇gi(xk)gi(xk)

  • Incremental gradient method:

ψi = ψi− 1 − αk∇gi(ψi− 1 )gi(ψi− 1 ), i = 1,... , m

ψ 0 = xk^ , xk+1^ = ψm

(ai x - bi )^2

x* mini^ a bii Advantage of incrementalism

R (^) maxi abi x i

CONJUGATE DIRECTION METHODS

  • Aim to improve convergence rate of steepest descent, without incurring the overhead of New- ton’s method
  • Analyzed for a quadratic model. They require n iterations to minimize f (x) = (1/2)x′Qx − b′x with Q an n × n positive definite matrix Q > 0.
  • Analysis also applies to nonquadratic problems in the neighborhood of a nonsingular local min
  • Directions d^1 ,... , dk^ are Q-conjugate, if di′Qdj^ = 0 for all i = j
  • Generic conjugate direction method: xk+1^ = xk^ + αkdk^ where the dks are Q-conjugate and αk is obtained by line minimization

y^0 y^1 y^2 w 0

w^1

x^0 x^1 x^2

d 0 = Q -1/2^ w^0

d^1 = Q -1/2w^1 Expanding Subspace Theorem

GENERATING Q -CONJUGATE DIRECTIONS

  • Given set of linearly independent vectors ξ^0 ,... , ξk, we can construct a set of Q-conjugate directions d^0 ,... , dk^ s.t. Span(d^0 ,... , di) = Span(ξ^0 ,... , ξi)
  • Gram-Schmidt procedure. Start with d^0 = ξ^0. If for some i < k, d^0 ,... , di^ are Q-conjugate and the above property holds, take

di+1^ = ξi+1^ +

i

m=

c(i+1)mdm;

choose c(i+1)m^ so di+1^ is Q-conjugate to d^0 ,... , di,

Qdj^ = 0.

i

m=

di+1′Qdj^ = ξi+1′Qdj^ + c(i+1)mdm

d^2 = ξ^2 + c^20 d^0 + c^21 d^1 d^1 = ξ^1 + c^10 d^0 ξ 2

d 1 d 0

ξ^1

0 (^0) - c (^10) d (^0) ξ (^0) = d 0

[ ][

QUASI-NEWTON METHODS

  • xk+1^ = xk^ − αkDk∇f (xk), where Dk^ is an inverse Hessian approximation
  • Key idea: Successive iterates xk, xk+1^ and gra- dients ∇f (xk), ∇f (xk+1), yield curvature info

qk^ ≈ ∇^2 f (xk+1)pk,

pk^ = xk+1^ − xk, qk^ = ∇f (xk+1) − ∇f (xk). ]− 1 ∇^2 f (xn) ≈ q^0 · · · qn−^1 p^0 · · · pn−^1

  • Most popular Quasi-Newton method is a clever way to implement this idea

Dk+1^ = Dk^ +

pkpk′ −

Dkqkqk′Dk

  • ξkτ kvkvk′^ , pk′^ qk^ qk′Dkqk

vk^ =

pk −

Dkqk , τ k^ = qk′Dkqk^ , 0 ≤ ξk^ ≤ 1 pk′^ qk^ τ^ k

and D^0 > 0 is arbitrary, αk^ by line minimization, and Dn^ = Q−^1 for a quadratic.

NONDERIVATIVE METHODS

  • Finite difference implementations
  • Forward and central difference formulas

∂f (xk) ≈

f (xk^ + hei) − f (xk) ∂xi^ h

∂f (xk) ≈

f (xk^ + hei) − f (xk^ − hei) ∂xi^2 h

  • Use central difference for more accuracy near convergence

xk

x k+1^ x

k+

  • Coordinate descent. Applies also to the case where there are bound constraints on the vari- ables.
  • Direct search methods. Nelder-Mead method.