# How to calculate an exact solution: non-linear equation with multiple variables

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JongGu Han on 10 Aug 2022
Edited: JongGu Han on 12 Aug 2022
In order to diagonalization of 3x3 matrix, I'm using 'fsolve' command. (there are 3 solutions for each matrix element of diagoanl)
However, it shows the deviation between the calculation solution and the real solution. Even it is reduced by change the initial value.
It means fsolver could show the exact value, I guess...
(I tried setting the option such as 'FunctionTolerance', 'StepTolerance', there was no effect.)
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v1=0;
T_eff=Coef(v1,1); A_eff=Coef(v1,2); eps_eff=Coef(v1,3); alp_eff=Coef(v1,4); B_eff=Coef(v1,5); D_eff=Coef(v1,6); p_eff=Coef(v1,7); q_eff=Coef(v1,8); A_D_eff=Coef(v1,9); gamma_eff=Coef(v1,10); H_eff=Coef(v1,11);
Eg=zeros(100,3);
for J=0:1:50;
x=N*(N+1);
W11=T_eff-A_eff+eps_eff-alp_eff+B_eff*(x+1)+0.25+q_eff*x+0.5*(1-1)*(q_eff+p_eff)-A_D_eff*(x+1)-D_eff*(x^2+4*x+1)-H_eff*(x^3+9*x^2+9*x+1);
W12=-(B_eff+(0.25-0.5)*q_eff+(0.125-1/4)*p_eff-0.5*A_D_eff-2*D_eff*(x+1)-0.5*gamma_eff+H_eff*(3*x^2+10*x-1))*sqrt(2*x);
W13=-(2*D_eff+0.25*q_eff-2*H_eff*(3*x-1))*sqrt(x*(x-2));
W21=W12;
W22=T_eff-2*eps_eff+B_eff*(x+1)+0.25*q_eff*(1-1)*x+0.5*(q_eff+p_eff)-D_eff*(x^2+6*x-3)+H_eff*(x^3+15*x^2-5*x+5);
W23=-(B_eff+0.25*q_eff+0.125*p_eff+0.5*A_eff-2*D_eff*(x-1)-0.5*gamma_eff+H_eff*(3*x^2-2*x+3))*sqrt(2*(x-2));
W31=W13;
W32=W23;
W33=T_eff+A_eff+eps_eff+B_eff*(x-3)+0.25*q_eff*(x-2)+A_D_eff*(x-3)-D_eff*(x^2-4*x+5)+H_eff*(x^3-3*x^2+5*x-7);
E_mat=@(E)det([W11-E(1) W12 W13; W21 W22-E(2) W23; W31 W32 W33-E(3)]);
E0=[W11,W22,W33];
options=optimoptions('fsolve','Display','iter','FunctionTolerance',0,'StepTolerance',0);
[E,fval]=fsolve(E_mat,E0,options);
E_ans(J+1,:)=det([W11-E_B W12 W13; W21 W22-E_B W23; W31 W32 W33-E_B]); %test line for checking the deviation
end
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No Solution Found
fsolve stopped because the problem appears regular as measured by the gradient, but the vector of function values is not near zero as measured by the default value of the function tolerance.
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determinant error is below just 1e-5, however, I want to exact value.

Torsten on 10 Aug 2022
Use "eig", not "fsolve".
JongGu Han on 12 Aug 2022
Edited: JongGu Han on 12 Aug 2022
oh, it's poor mistake.
Thank you.

R2020b

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