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Matlab giving the warning of Exiting fzero!

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Dhawal Beohar
Dhawal Beohar 2022-3-9
编辑： Torsten 2022-3-15
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);
  1 个评论
Walter Roberson
Walter Roberson 2022-3-9
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

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采纳的回答

Walter Roberson
Walter Roberson 2022-3-9
Ran in:
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 = 
  8 个评论
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Dhawal Beohar
Dhawal Beohar 2022-3-15
Thanks, How it can be done using fzero? It is giving answer as NaN
Torsten
Torsten 2022-3-15
编辑：Torsten 2022-3-15
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 ?

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