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**benice****Member**- Registered: 2010-06-10
- Posts: 110

Hi,

The MathsIsFun Equation Grapher gives wrong graph for

(abs(x-1) + abs(y) - 1) * (abs(x) + abs(y-1) - 1) = 0.

*Last edited by benice (2011-08-10 16:17:48)*

My Gallery of Math Art: benice equation

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 84,526

Hi benice;

Mathematica agrees with you. I remember trying to write my own contour plotter. In two different languages! I do not think I ever really succeeded.

I tried to do it by solving for the points. That is difficult enough when you are standing over the computer and watching every move. In a plotter the program has to do it all by itself. Sometimes they are going to miss some and sometimes they will add a few.

By the way that is a clever way to draw two squares.

**In mathematics, you don't understand things. You just get used to them.I have the result, but I do not yet know how to get it.All physicists, and a good many quite respectable mathematicians are contemptuous about proof.**

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,535

Hi benice,

Thanks for that, I will look into it.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**benice****Member**- Registered: 2010-06-10
- Posts: 110

Hi bobbym,

I agree with you. It's difficult to write a contour plotter from scratch.

Many thanks to those kind people who make open/free plotters for us.

My Gallery of Math Art: benice equation

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**benice****Member**- Registered: 2010-06-10
- Posts: 110

Hi MathsIsFun,

I have found this is a common problem in many open softwares including Graph, gnuplot and Scilab.

Maybe **this page** will be helpful to you.

My Gallery of Math Art: benice equation

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 84,526

Hi benice;

Graph, gnuplot and Scilab

That is some good software there. I do not feel so bad now. I am not sure about his explanation of no sign changes.

y - x^2 = 0 has no sign changes either but graphs properly here. Or maybe you do not notice it because only (0,0) is left out!

**In mathematics, you don't understand things. You just get used to them.I have the result, but I do not yet know how to get it.All physicists, and a good many quite respectable mathematicians are contemptuous about proof.**

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,535

Yes, sign changing abruptly back and forth is hard to detect. This will be an interesting test case for me.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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