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cberceanu1-Visitor
1-Visitor
October 6, 2014
Solved

effect of guess values on "minimize" function

  • October 6, 2014
  • 3 replies
  • 1238 views

Hi,

I have two different files in which I have the same function defined. For this function, I want to perform a "minimize" in order to find out its minimum across a certina interval of the variables that make up the function. I would have expected the guess values to have no effect on the rezult of the minimize, but apparently they do have (I get different results from applying "minimize" depending on the guess values I write. Does anybody know whty this happens? Below you may find a screenshot of the two different mathcad files, side by side. I have also attached the files themselves.

Thank you and best regards,

Cristian

Untitled.png

Best answer by Werner_E

Here is a better view of the surface you try to get a minumum of. This time I plotted each "part" of it as a separate plot to avoid the artefacts inbetween.

You clearly see the asymptotic bahaviour.

3.png

Accidentially when I just wanted to add the new 3D plot picture I deleted my previous post where I went into detail why the effect you experience is normal behaviour given the numerical algorithm and especially the specific function you try to minimize.

But by lookng at the plot you sure get the idea why it doesn't make much sense to search for a minimum of this function. You can get as close to minus infinity if you stay near the line y=1/3*(x+1). For example let x=5 and y=2.00000000001 or x=2 and y=1.000000000001.

The values you get by minize roughly follow that line, too, of course.

3 replies

Werner_E
Werner_E25-Diamond IAnswer
25-Diamond I
October 6, 2014

Here is a better view of the surface you try to get a minumum of. This time I plotted each "part" of it as a separate plot to avoid the artefacts inbetween.

You clearly see the asymptotic bahaviour.

3.png

Accidentially when I just wanted to add the new 3D plot picture I deleted my previous post where I went into detail why the effect you experience is normal behaviour given the numerical algorithm and especially the specific function you try to minimize.

But by lookng at the plot you sure get the idea why it doesn't make much sense to search for a minimum of this function. You can get as close to minus infinity if you stay near the line y=1/3*(x+1). For example let x=5 and y=2.00000000001 or x=2 and y=1.000000000001.

The values you get by minize roughly follow that line, too, of course.

C
cberceanu1-VisitorAuthor
1-Visitor
October 7, 2014

Thanks!

Do not worry, I did manage last evening to read your initial post and that explained it all (I just did not have the time to reply then). I understand that this particular function is just an incorrectly selected example to prove the functionality of minimize and maximize so I will address that. Thank you again!

Best regards,

Cristian

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