fminbnd finding wrong minimum point for no reason, please help

f = @(x)(-(1.0/((x-0.3).^2+0.01)+1.0/((x-0.9).^2+0.04)));
x = fminbnd(f, -1, 2);
x
y=f(x);
y
x = -1:0.3:2
y = -(1./((x-0.3).^2+0.01)+1./((x-0.9).^2+0.04))
plot(x,y)

here, minimum of the function should be smt between -50 and -60, but it finds -102.5014!! Why is that? please help.

Answer 1

[Andrei Bobrov]

f = @(x)(-(1.0./((x-0.3).^2+0.01)+1.0./((x-0.9).^2+0.04)));
x1 = fminbnd(f, -1, 2);
x1
y=f(x1);
y
x = sort([-1:0.3:2,x1])
y = -(1./((x-0.3).^2+0.01)+1./((x-0.9).^2+0.04))
plot(x,y)

[Emre]

thank you very much but why was my plot wrong?

[Andrei Bobrov]

due to the choice of x

[Andrei Bobrov]

use

ezplot(f,[-1,2,f(x1)-.2,0])

Answer 2

[Walter Roberson]

As was indicated more than once in resposne to your previous question, you are trying to apply fminbnd() to a problem with multiple minima, but fminbnd() is not designed to handle such situations.

If you want to handle functions like the one above, you need a global minimizer.