Interior Point 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: Interior Point Methods, Barrier, Linear Programs, Logarithmic Barrier, Inequality Constrained Problem, Minimize, Continuous, Set, Feasiblepoint, Approaches

Typology: Slides

2012/2013

Uploaded on 03/27/2013

ekana
ekana 🇮🇳

4

(44)

370 documents

1 / 8

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
NONLINEAR PROGRAMMING
LECTURE 15: INTERIOR POINT METHODS
LECTURE OUTLINE
Barrier and Interior Point Methods
Linear Programs and the Logarithmic Barrier
Path Following Using Newton’s Method
Inequality constrained problem
minimize f(x)
subject to x X, gj
(x) bj
,j =1,...,r,
where f and gj are continuous and X is closed.
We assume that the set
S = x X | gj
(x) < 0,j =1,...,r
isnonemptyandanyfeasiblepointisintheclosure
of S.
Docsity.com
pf3
pf4
pf5
pf8

Partial preview of the text

Download Interior Point Methods - Nonlinear Programming - Lecture Slides and more Slides Computer Science in PDF only on Docsity!

NONLINEAR PROGRAMMING

LECTURE 15: INTERIOR POINT METHODS

LECTURE OUTLINE

• Barrier and Interior Point Methods

• Linear Programs and the Logarithmic Barrier

• Path Following Using Newton’s Method

Inequality constrained problem

minimize f (x)

subject to x ∈ X, gj (x) ≤ bj , j = 1,... , r,

where f and gj are continuous and X is closed.

We assume that the set

S = x ∈ X | gj (x) < 0 , j = 1,... , r

is nonempty and any feasible point is in the closure

of S.

BARRIER METHOD

• Consider a barrier function, that is continuous

and goes to ∞ as any one of the constraints gj (x)

approaches 0 from negative values. Examples:

r r 1 B(x) = − ln −gj (x) , B(x) = −. gj (x) j=1 j=

• Barrier Method:

xk^ = arg min f (x) + �k^ B(x) , k = 0, 1 ,... , x∈S

where the parameter sequence {�k^ } satisfies 0 <

�k+1^ < �k^ for all k and �k^ → 0.

Boundary of S Boundary of S

ε B(x)

ε' B(x)

ε' < ε

S

LINEAR PROGRAMS/LOGARITHMIC BARRIER

• Apply logarithmic barrier to the linear program

minimize c′^ x

(LP)

subject to Ax = b, x ≥ 0 ,

The method finds for various � > 0 ,

c ,

n

i=

x(�) = arg min^ ′^ x − � x∈S

F�(x) = arg min x∈S

ln xi

x | Ax = b, x > 0 }. We assume that S is

nonempty and bounded.

where S =

• As � → 0 , x(�) follows the central path

All central paths start at Point x(ε) on central path

x∞

S

x* (ε = 0)

c

and end at optimal solu tions of (LP).

the analytic center

n

i=

x∞ = arg min x∈S

ln xi

PATH FOLLOWING W/ NEWTON’S METHOD

• Newton’s method for minimizing F�:

x ˜ = x + α(x − x),

where x is the pure Newton iterate

x†= arg min ∇F�(x)′(z†− x) + 12 (z†− x)′∇^2 F�(x)(z†− x)

Az=b

• By straightforward calculation

x = x − Xq(x, �),

Xz q(x, �) = − e, e = (1... 1)′^ , z = c − A′λ,

λ = (AX^2 A′)−^1 AX Xc − �e ,

and X is the diagonal matrix with xi, i = 1,... , n

along the diagonal.

• View q(x, �) as the Newton increment (x−x) trans-

formed by X−^1 that maps x into e.

• Consider ‖q(x, �)‖ as a proximity measure of the

current point to the point x(�) on the central path.

SHORT STEP METHODS

Following approximately the

Central Path central path by using a sin

x

Set {x | ||q(x,εk^ )|| < 1}

x∞

x(εk+1^ ) xk x(εk^ )

xk+

Set {x | ||q(x,εk+1^ )|| < 1}

S

gle Newton step for each �k^. If �k^ is close to �k+ and xk^ is close to the cen tral path, one expects that xk+1^ obtained from xk^ by a single pure Newton step will also be close to the central path.

Proposition Let x > 0 , Ax = b, and suppose that

for some γ < 1 we have ‖q(x, �)‖ ≤ γ. Then if � =

(1 − δn−^1 /^2 )� for some δ > 0 ,

‖q(x, �)‖ ≤

γ^2 + δ 1 − δn−^1 /2

In particular, if

δ ≤ γ(1 − γ)(1 + γ)−^1 ,

we have ‖q(x, �)‖ ≤ γ.

• Can be used to establish nice complexity results;

but � must be reduced VERY slowly.

LONG STEP METHODS

• Main features:

− Decrease � faster than dictated by complex-

ity analysis.

− Require more than one Newton step per (ap-

proximate) minimization.

− Use line search as in unconstrained New-

ton’s method.

− Require much smaller number of (approxi-

mate) minimizations.

x^ * * Central Path

x∞

x(εk+1^ ) x k x(εk^ )

xk+

x k+2 x(εk+2^ )

x

S

Central Path

x∞

x(εk+1^ ) xk x(εk^ )

xk+

x (^) x(εk+2 (^) ) k+

S

(a) (b)

• The methodology generalizes to quadratic pro-

gramming and convex programming.