X5 = - PowerPoint Presentation

Slide1

X5 = 5 - 1 X1 + 2 X2 - 1 X3 + 4 X4 X6 = 10 + 0 X1 - 1 X2 + 1 X3 + 0 X4 X7 = 3 + 1 X1 + 0 X2 + 0 X3 + 5 X4---------------------------------------z = 0 + 10 X1 + 10 X2 +25 X3 + 1 X4

You only have 5 minutes left to finish your first test.Which variable would you choose to be the entering variable and why to quickly solve this problem?

1Slide2

Assignment #1: due today at the beginning of class (or Monday at the beginning of class for a 10% late penalty).Any questions?2Slide3

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Last class: degenerate pivoting caused an infinite loop.Theorem [Bland, 1977] The Simplex method does not repeat dictionaries (and hence terminates) as long as both the entering and leaving variables are chosen by the smallest-subscript rule in each iteration.4Slide5

Note about reading a mathematical proof: It’s not like reading a novel.You sometimes have to read a proof very slowly writing down and checking every step as you go.It took me a long time to believe and follow the next proof.Fortunately, most of the proofs in this class are not this tedious.5Slide6

We need to show cycling does not happen.Suppose it does:D0, D1, D2, D3, … , Dk=D0, D1, D2, D3, … , Dk=D0,…Fickle variable: basic in some of these dictionaries and non-basic in others.xt = fickle variable with largest subscript.Idea of proof: Argue that when xt leaves, some variable xr with r < t was eligible to leave and should have left instead.

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Pivot from dictionary D: xt leaves and xs enters (xt is basic in D but not in the next dictionary).Pivot from dictionary D*: xt enters again.Technical point:Since the loop is cyclic:D0, D1, D2, D3, … , Dk=D0, D1, D2, D3, … , Dk=D0,

D= Di, and D* = D

j

but we may have j <

i

.

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Dictionary D (xt leaves and xs enters) : xi = bi - for each i ∈ B. -----------------------------------------------------------z = v + Since all the pivots are degenerate, the

last row of D* has the same constant term v.The last row of D* (including all variables):z

=

v

+

Set

c

j

* = 0 if

x

j

is basic in D*.

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Dictionary D:xi = bi - for each i ∈ B. ----------------------------------------------------------z = v + One solution to D: Set non-basic variable (the entering one) xs= y and the other non-basic ones to 0 and get the rest from the dictionary:xi = bi - a

is y for each i

∈ B.

-----------------------------------------------------------

z = v +

c

s

y

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Set non-basic variable (the entering one) xs= y and the other non-basic ones to 0 and get the rest from the dictionary:xi = bi - ais y for each i ∈ B. -----------------------------------------------------------z = v + cs yPlug this into equation for z from D*:z = v +

z = v +

c

s

*

y

+

b

i

-

a

is

y

)

The other variables have value 0 so I have not included them in the z equation here.

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From D: z = v + cs yFrom D* (same value for z):z = v + cs* y + bi - ais y )So:v + cs y = v + cs* y +

b

i

-

a

is

y

)

Simplify algebraically:

(

c

s

-

c

s* +

ais

) y =

b

i

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(cs - cs* + ais ) y = biThe RHS is a constant that does not depend on y! So how can this be satisfied for all y?Only when:(cs - cs* +

ais

) = 0

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(cs - cs* + ais ) = 0Since xs enters when we pivot from D:cs > 0. Since xs does not enter when we pivot from D* (xt enters and t > s, so xs is not eligible to enter), cs* ≤ 0.Therefore cs - cs* > 0. H

ence for at least one value of r with r ∈ B, cr*

a

rs

< 0.

Since r

∈ B,

x

r

is basic in D.

Since

c

r

*

is not 0

,

xr is not basic in D*.Hence, xr is fickle and r ≤ t.

 13Slide14

Can we have xr = xt? No!xt leaves from D and xs enters.This means that D has a row like this:xt= 0 - ats xs + ….where we must have ats > 0.Since t enters from D*, ct* > 0.Therefore, ct* a

ts > 0.

But recall that we have:

c

r

*

a

rs

< 0.

Therefore r and t are different.

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Now r< t and r does not enter in D*.Therefore cr* ≤ 0.But recall that we have: cr* ars < 0.So ars > 0.Fickle variables stay 0 over all the degenerate pivots: if one of them increased then z would be able to increase. So xr=0 in D and D*.D must have had a row like this:xr= 0 – ars xs + … with ars > 0so r should have exited from D instead of t when s entered.

This is the contradiction we need to end the proof.

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Observation (from the proof):If you use smallest subscript rule to decide on both the entering and exiting variables:Consider all the variables that are fickle in the degenerate pivots: once the one with maximum subscript leaves then it is not permitted to come back again until you havesome non-degenerate pivot.16