# Problem with NLSolve: cant find x st f(x) =0
**URL:** https://discourse.julialang.org/t/problem-with-nlsolve-cant-find-x-st-f-x-0/88345
**Category:** Specific Domains
**Tags:** nlsolve
**Created:** [October 6, 2022, 1:53pm UTC](https://discourse.julialang.org/t/problem-with-nlsolve-cant-find-x-st-f-x-0/88345 "2022-10-06T13:53:57Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Outeirom](https://discourse.julialang.org/u/Outeirom)
#### Post date: [October 6, 2022, 1:53pm UTC](https://discourse.julialang.org/t/problem-with-nlsolve-cant-find-x-st-f-x-0/88345/1 "2022-10-06T13:53:57Z")
So, I have a code in matlab and I am trying to translate it to Julia.
Im not being able to run this NLSolve in Julia. It returns me a domain error… probably because there is x^eta, where eta = 0.5.
But what makes no sense for me is that it runs in matlab. I have even tried to use ’ fzero ’ in Julia, and it still does not work: returns domain problem (but with different error outcome).
I would not say there is an error in my function (PrivateF) itself because when I run only this function with arbitrary values, both Julia and Matlab returns the same value. For example, if x=10, both matlab and julia results are equal.
Therefore, I think the problem is in the optimization part of NLsolve / fzero (roots package).
The code is just below, both for matlab and Julia. If anyoen could help me, I would be extremaly grateful.
```julia
#Parameters
beta=0.5
r=0.0040;
tauu=0.0019;
kappa=14.1425;
zeta=1;
eta=0.5;
b=0.7395;
y=1.741;
s_p=0.007382165;
#%% Matlab
function F = PrivateF(x, kappa , zeta , eta , beta , y , b , s_p, rr, tauu)
F = [kappa *x(1)^(1-eta ) /zeta - (1-beta )*(y -b )/(rr+tauu+s_p+beta *zeta *x(1)^eta )];
#
#% Solving system in Matlab
[theta ,fval,exitflag] = fsolve(@(x) PrivateF(x,kappa , zeta , eta , beta , y , b , s_p, r, tauu), x0 ,optimset('Display','off', 'TolFun',1.0e-009));
# end matlab code
### Julia
function PrivateF(x, kappa , zeta , eta , beta , y , b , s_p, rr, tauu)
F = [kappa *(x[1])^(1-eta ) / zeta - (1-beta ) * (y -b )/(rr+tauu+s_p+beta *zeta *x[1]^eta )];
return F
end
#Solving system
x0 = [10.0]; #starting guess
f_aux = (x -> PrivateF(x, kappa , zeta , eta , beta , y , b , s_p, r, tauu))
theta = nlsolve(f_aux, x0).zero
#even tried with fzero
theta = fzero(f_aux, x0[1])
```
Thank you very much!
---
### Author:  [@josuagrw](https://discourse.julialang.org/u/josuagrw)
#### Post date: [October 6, 2022, 2:00pm UTC](https://discourse.julialang.org/t/problem-with-nlsolve-cant-find-x-st-f-x-0/88345/2 "2022-10-06T14:00:43Z")
It looks like your function is only defined for positive `x` — so maybe [`mcpsolve()`](https://github.com/JuliaNLSolvers/NLsolve.jl#mixed-complementarity-problems) (from `NLsolve.jl`) is a better method for your case.
---
### Author:  [@josuagrw](https://discourse.julialang.org/u/josuagrw)
#### Post date: [October 6, 2022, 2:06pm UTC](https://discourse.julialang.org/t/problem-with-nlsolve-cant-find-x-st-f-x-0/88345/3 "2022-10-06T14:06:37Z")
Does this work for you?
```julia
julia> theta = nlsolve(x -> f_aux(max.(0.0, x)), x0).zero
1-element Vector{Float64}:
0.06408989836281948
```
---
### Author:  [@Outeirom](https://discourse.julialang.org/u/Outeirom)
#### Post date: [October 6, 2022, 2:18pm UTC](https://discourse.julialang.org/t/problem-with-nlsolve-cant-find-x-st-f-x-0/88345/4 "2022-10-06T14:18:53Z")
Thank you very much!!! Exactly the same outcome that I had in Matlab.
I didnt know this (max.(0.0,x)) formulation.
---
### Author:  [@josuagrw](https://discourse.julialang.org/u/josuagrw)
#### Post date: [October 6, 2022, 4:39pm UTC](https://discourse.julialang.org/t/problem-with-nlsolve-cant-find-x-st-f-x-0/88345/5 "2022-10-06T16:39:52Z")
The error your original code produces, indicates that your (real) function `PrivateF` is defined only on the non-negative real axis. `max(0, x)` insulates `PrivateF` from any negative `x` the root-finding algorithm throws at it.