Hello all,

I have questions related to calculating the min and max of set of variables. First I need to calculate the max(x1,x2,x3) and then the min(x1,x2,x3) after that I want to minimize the difference between the max and the min. How do we formulate and linearize such problem (in the constraints)?

Thanks

asked 31 May '16, 13:32

hdauod1's gravatar image

hdauod1
112
accept rate: 0%

edited 31 May '16, 13:33

1

What is the context of the model? Is minimizing the difference the only thing in the objective? Do you need to know the max and/or min for any purpose in the model other than the objective?

(31 May '16, 15:47) Paul Rubin ♦♦

Its a scheduling problem for orders. for each order i, I want to calculate the difference between max{c1,c2,c3} and min{c1,c2,c3} where c is the job completion time. After that I want to minimize the total sum of differences of all orders (sum i in orders) diff[i]

so I only need this difference in objective function

Thank you

(31 May '16, 16:25) hdauod1

min z - y

y <= x1 <= z

y <= x2 <= z

y <= x3 <= z

link

answered 01 Jun '16, 04:20

opti100's gravatar image

opti100
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edited 01 Jun '16, 04:20

This only gives me the max but it doesnt work for the min. When I run it gives me that y=z, which is not true.

(01 Jun '16, 10:02) hdauod1
1

But this is the correct version for what you requested. Take a look at the values of the variables x1, x2, x3, you might have an error somewhere.

(01 Jun '16, 11:11) opti100

Can you use integer variables ? It's very straightforward using them.

max(x1,x2,x3):

real var max; binary var not1, not2, not3;

max >= x1; max >= x2; max >= x3;

max <= x1 + bigM * not1; max <= x2 + bigM * not2; max <= x3 + bigM * not3;

bigM is a constant always >= any possible value for {x1, x2, x3}.

not1 + not2 + not3 = 2 (you are cancelling just 2 of these 3 constraints).

Without using IP and using only linear constraints I don't think it's possible.

link

answered 03 Jul '16, 11:32

Rafael%20Arakaki's gravatar image

Rafael Arakaki
112
accept rate: 0%

edited 03 Jul '16, 11:32

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Asked: 31 May '16, 13:32

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Last updated: 03 Jul '16, 11:32

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