Error solving for a particular variable from two equations

Hi all,

I am having trouble with solving for a particular variable 'a' from two equations, I end up with an error saying :

Array indices must be

positive integers or

logical values.

Error in sym/privsubsasgn

(line 1124)

L_tilde2 =

builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);

Error in sym/subsasgn

(line 961)

C =

privsubsasgn(L,R,inds{:});

Error in assignment3 (line

10)

sig_on_sigo_real(TonTc)=sig_on_sigo_2;

From the following code:

clc, clear, clear all

kB=-1.381*10^-16;

mu_H=9.274*10^-21;

J_value=0.5

syms a %Should I do this?

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); %First equation

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

solve(sig_sigo_1, sig_on_sigo_2); %Trying to solve for a

sig_on_sigo_real(TonTc)=sig_on_sigo_2; %subbing in the value of a I obtained from the line before

end

What should I do to properly solve for 'a'?

Cheers

Answer 1

[Alan Stevens]

You could use fzero:

TonTc = 0.01:0.01:0.99;
a = zeros(1,numel(TonTc));

a0 = 1; % initial giuess
for k = 1:numel(a)
    
    a(k) = fzero(@(a) fn(a,TonTc(k)),a0); 
    
end

plot(TonTc,a,'o'),grid
xlabel('TonTc'),ylabel('a')

function z = fn(a, TonTc)

         J_value = 0.5;
         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;
end