Unable to solve for a variable

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Jonas Freiheit 2021-10-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1468006-unable-to-solve-for-a-variable

评论： Jonas Freiheit 2021-10-10

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

Hi all,

I am trying to solve for a variable 'a' and saving it into an array, However, I am left with an inifinitely long running program. What should I do?

Heres a section of the code:

clc, clear, clear all
J_value=0.5
syms a;
for TonTc=.01:0.01:.99
    sig_sigo_1=((2*J_value+1)/2*J_value)*coth((2*J_value+1)/2*J_value)*a-(1/2*J_value)*coth(a/2*J_value); 
    sig_on_sigo_2=((J_value+1)/3*J_value)*TonTc*a; 
    z=sig_sigo_1-sig_on_sigo_2;
    for k=1:99
        val_a(k)=vpasolve(z,a);
    end
end

Cheers

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采纳的回答

Walter Roberson 2021-10-7

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1468006-unable-to-solve-for-a-variable#answer_803216

在 MATLAB Online 中打开

J_value=0.5;

syms a;

TonTc_vals = .01:0.01:.99;

num_Tonc = length(TonTc_vals);

val_a = zeros(num_Tonc, 1);

for k = 1 : num_Tonc

TonTc = TonTc_vals(k);

sig_sigo_1=((2*J_value+1)/2*J_value)*coth((2*J_value+1)/2*J_value)*a-(1/2*J_value)*coth(a/2*J_value);

sig_on_sigo_2=((J_value+1)/3*J_value)*TonTc*a;

z=sig_sigo_1-sig_on_sigo_2;

sol = vpasolve(z, a, -1);

if isempty(sol)

val_a(k) = nan;

else

val_a(k) = sol(1);

end

if k == 1

fplot([z, 0], [0.5 1])

title("plot for TonTc = " + TonTc)

ylim([-2 2])

end

figure

plot(TonTc_vals, val_a)

title("a vs TonTc")

That first plot makes it clear that there is no root at 0.75

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Walter Roberson 2021-10-10

在 MATLAB Online 中打开

Yes, there are two solutions.

J_value=0.5;

syms a;

TonTc_vals = .01:0.01:.99;

num_Tonc = length(TonTc_vals);

val_a = zeros(num_Tonc, 1);

for k = 1 : num_Tonc

TonTc = TonTc_vals(k);

sig_sigo_1=((2*J_value+1)/(2*J_value))*coth((2*J_value+1)/(2*J_value))*a-(1/(2*J_value))*coth(a/(2*J_value));

sig_on_sigo_2=((J_value+1)/(3*J_value))*TonTc*a;

z=sig_sigo_1-sig_on_sigo_2;

sol = vpasolve(z, a, -1);

if isempty(sol)

val_a(k) = nan;

else

val_a(k) = sol(1);

end

if k == 1

fplot([z, 0], [-2 2])

title("plot for TonTc = " + TonTc)

ylim([-2 2])

end

figure

plot(TonTc_vals, val_a)

title("a vs TonTc")

Jonas Freiheit 2021-10-10

Yes, Sorry How would I plug in the second solutions?

Just trying to test to see if that gives me the theoretical results. This is the equation I'm supposed to follow to obtain sigma/sigma0 which I think I've correctly used in the code.