Syms equation in GUI to @ equation solver

Hello, I write a GUI on Matlab 2020 with all the Toolbox, but need to work also on a Matlab 2007 without all the Toolbox.

Currently I use :

function [dt] = DeltaT(lambda0,c,deltaLambdaSPM,tfwhm)
dtlim=lambda0/2/c %% lower limit
syms x
A=c*lambda0*2*x/(2*c*x-lambda0);
B=c*lambda0*2*x/(2*c*x+lambda0);
C=A-B;
eqn=C==7*deltaLambdaSPM;

varLimits(1,1) = dtlim;
varLimits(2,1)=tfwhm/5 ;
S = vpasolve([eqn],x,varLimits);
dt=double(S);
end

With Matlab 2020 + Toolbox it's working, not with the other.

So I try to modify it

lambda0=1030e-9;
c=3e8;
tfwhm=500e-15;
deltaLambdaSPM=1.35e-06;
dtlim=lambda0/2/c;
fun = @(x) c*lambda0*2*x./(2*c*x-lambda0)- c*lambda0*2*x./(2*c*x+lambda0) - 7*deltaLambdaSPM
x0 = [dtlim tfwhm/5];
z = fzero(fun,x0)

%%

Error using fzero (line 229)

Function values at interval

endpoints must be finite and

real.

Error in testesuationsolver

(line 17)

z = fzero(fun,x0)

I try also fsolve...

solution should be : 1.9e-15

If someone have an idea?

Regards

[Asvin Kumar]

fzero(dtlim) is Inf.

[Martin Maurel]

Your are right,

x0 = [1.00000001*dtlim tfwhm/5];

solve it