how can i plot the values of 2 variables against eachother from the determinant of a martix when det ==0?

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Seb apple 2020-6-16

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评论： Seb apple 2020-6-16

I am trying to plot the answers for variables o and e from my code when the det of my matrix ==0. when i use solve i get struct results back that i cannot seem to plot.

syms e o n
c=0;
g(n)=e/(2*(o+(1i*n*c)-((n)^2)));
 
f=[1 g(3) 0 0 0 0 0; g(2) 1 g(2) 0 0 0 0; 0 g(1) 1 g(1) 0 0 0; 0 0 g(0) 1 g(0) 0 0; 0 0 0 g(-1) 1 g(-1) 0; 0 0 0 0 g(-2) 1 g(-2); 0 0 0 0 0 g(-3) 1];
F=det(f)==0;
 
solo=solve(F, o, 'ReturnConditions', true);
sole=solve(F, e, 'ReturnConditions', true);
 

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KSSV 2020-6-16

Read about fplot.

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Answer 1

John D'Errico 2020-6-16

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编辑：John D'Errico 2020-6-16

在 MATLAB Online 中打开

First, need I point out just how bad of an idea is it to name a variable a lower case o? This is just asking to produce buggy code, when you misread or mistype o as 0.

Regardless, Just set

F = det(F);

If you look at F, you will see it is essentially a high degree (greater than 4 is all that matters) polynomial. So algebraic solutions will often be difficult to find.

Then use fimplicit to plot the solution locus. Settign it to zero does not help. In fact that will cause problems, and fimplicit does that implicitly anyway.