Check if function handle is nonlinear with Matlab

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Daniela Würmseer
评论： Torsten 2022-4-3
Hello,
is it possible to check with matlab if a Function Handle is nonlinear?
For example:
f1=@(x) x(1)+x(2)-5;
f2=@(x)10*x(1)-x(1)^2+4*x(2)-x(2)^2;
So here i would need something which tells me that f1 is linear and f2 is nonlinear. Thanks
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Paul
Paul 2022-4-2
The first function is linear in x, I suppose, but is not linear from a system theoretric perspective, as it doesn't satisfy superposition or scaling. Just want to make sure we're clear on the definition of linear in the question.

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Torsten
Torsten 2022-4-2
编辑：Torsten 2022-4-2
Call your function handle f with symbolic vectors x and y and a symbolic scalar a and check whether
f(x+y) - (f(x) + f(y)) - f(0) == 0
f(a*x) - a*f(x) == 0
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Daniela Würmseer
Daniela Würmseer 2022-4-3
Thank you for your answers.
Perhaps i could solve my Problem differently: My Problem is that i have an Algorithm which takes an Optimization Problem with two functions f1, f2 which can be linear or nonlinear. And the Algorithm solves this Problem by a kind of direction Method. So it solves first a "new" Optimization Problem with one function (max alpha) and the constraints are the constraints of the origin Problem and we add as constraints for example: (those are only for illustration and not the real algorithm):
f1(x)-alpha <= 0 and f2(x)-alpha <=0.
I could just define those functions in nonlincon of fmincon but nonlincon is normally for nonlinear functions. Would it be a problem to use nonlincon with linear functions (normally not or)?
For example if f1 and f2 are linear to still use nonlincon like this:
%Definitions of f, x0, Aneq.....
nonlincon = @constr;
function [cneq,ceq] = constr(x)
ceq = [];
cneq(1) = f1(x)-alpha;
cneq(2) = f2(x)-alpha;
end
options = optimoptions('fmincon','Algorithm','sqp');
[x,alpha]=fmincon(f,x0,Aneq,bneq,Aeq,beq,lb,[],nonlincon,options);
Torsten
Torsten 2022-4-3
I could just define those functions in nonlincon of fmincon but nonlincon is normally for nonlinear functions. Would it be a problem to use nonlincon with linear functions (normally not or)?
No problem. Implementing linear constraints either in lb,ub or A, Aeq, b, beq should only make the code run faster.

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