How to make sure that fsolve gives only positive values as the solution

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kalpana
kalpana 2014-11-27
回答： Ninad Mauskar 2019-6-18
I have L equations to solve. I am using the following code to solve :
fh = matlabFunction([equn(1:L)],'vars',{[lam(1:L)]});
options = optimset('Display','off','TolFun',eps);
[solu_fsolve,value] = fsolve(fh, [1:L] ,options ) ;
But, i am getting -ve answer as a solution of fsolve. But, i am expecting + ve solutions only. Is there anyway to make sure that fsolve gives only +ve solutions?
Please answer. Thanks in advance.
With best regards, kalpana

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采纳的回答

Torsten
Torsten 2014-11-27
Square your unknowns in the functions you supply to fsolve.
E.g. if you want to solve
x(1)-5=0,
solve instead
y(1)^2-5=0.
After you get the solution y(1) from fsolve (in this case sqrt(5)), you only have to square it to get x(1) (in this case 5) - the solution of your original untransformed problem.
Best wishes
Torsten.

更多回答（1 个）

Ninad Mauskar
Ninad Mauskar 2019-6-18
Hi Kalpana,
Add a couple of conditional clauses before the return line.
For example, I am looking to solve for x such that array1[0,0] which is determined using x = certain fixed value.
I added a conditional statement before the return line which states:
if x < 0:
array1[0,0] = 0
else:
array1[0,0] is calculated using a specified formula which takes x as an input.
I have a return line after the above lines:
return array1[0,0] - certain value
When i use fsolve now, fsolve only gives be positive values of x.
Hope this helps!
Cheers,
Ninad

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