# Matlab giving the warning of Exiting fzero!

1 view (last 30 days)
Dhawal Beohar on 9 Mar 2022
Edited: Torsten on 15 Mar 2022
Exiting fzero: aborting search for an interval containing a sign change
because complex function value encountered during search.
(Function value at -2.8 is 10.0875+1.122i.)
Check function or try again with a different starting value.
I am getting the complex value for any initial value i am giving. How can i solve this issue. I have tried using different values for initial guess and changing some of my input values as well, but no success. Here is my code below. Any help would be really appreciated.
function fval = fun(u)
d1=20;
n=10^-11.4;
m=2.7;
a=0.5;
T=1;
PsByN_0dB=25;
PsByN_0=10.^(PsByN_0dB/10);
fun1 = @(u) (((-1./u)*log((d1^m)./(a*n*PsByN_0*T*u)+d1^m)*a*T)./(1-a)*T);
initialguess = 10;
zero = fzero(fun1,initialguess);
Walter Roberson on 9 Mar 2022
Nothing in your code assigns to fval .
Your code ignores the u that was passed in to fun. The u in the @(u) definition of fun1 "shadows" the u that was passed in to fun

Walter Roberson on 9 Mar 2022
d1=20;
n=10^-11.4;
m=2.7;
a=0.5;
T=1;
PsByN_0dB=25;
PsByN_0=10.^(PsByN_0dB/10);
fun1 = @(u) (((-1./u)*log((d1^m)./(a*n*PsByN_0*T*u)+d1^m)*a*T)./(1-a)*T);
syms U
F = fun1(U)
F = In order for that to be 0, either is infinite or the log() has to be 0 . But for the log() to be 0, the value inside would have to be 1
inner = children(children(F, 2),1)
inner = Ubest = solve(inner == 1)
Ubest = fun1(Ubest)
ans =
0
vpa(Ubest)
ans = Torsten on 15 Mar 2022
Edited: Torsten on 15 Mar 2022
Then you'll have to vary the initial u0 until you get a reasonable solution.
If it works with "fsolve", why don't you use it ?

R2020a

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!