Bug fminbnd not working

fplot(@(x) x*(sin(x))^2*cos(x),[-2*pi 2*pi]);
[xMin1 fvalmin1] =  fminbnd('-x*(sin(x))^2*cos(x)', -6, 6)
returns xMin1 = 1.0954
fvalmin1 = -0.3963 

How is this possible, look at the plot?

[madhan ravi]

Why do you say it's a bug?

[Stephen Wilkerson]

Not where the minimum is! look at the plot

[Stephen Wilkerson]

Solved, The function doesn't really do much, it give you the nearest point that's a minimum to it's starting point. In my case it woud be 0 as the starting point. Typical

Answer 1

[Walter Roberson]

No bug. fminbnd is a local minimizer, not a global minimizer.

[Zhe Yu]

Hi Roberson,

I have a similar problem with fminbnd, see code below

a=-200; b=-979.8997; c=7.1833e+05; d=24.4232;e=-6.6083;
x1=0; x2=4.1135e-06;
f=@(x)a-(a-b)*cos(c*(x-x1)) + d*e*sin(c*(x-x1))

f = function_handle with value:
    @(x)a-(a-b)*cos(c*(x-x1))+d*e*sin(c*(x-x1))

fplot(f, [x1 x2])

[xmin, min]=fminbnd(f, x1, x2)

xmin = 1.5712e-06

min = -679.5775

We could see from the plot the minimum value should be around -996 and there should be only one local minimum. But fminbnd returns -679.

Thanks a lot in advance!

Best Regards,

Zhe

[Torsten]

Since the changes in the x-values are in the order of 1e-6, you must choose a smaller value for TolX:

a=-200; b=-979.8997; c=7.1833e+05; d=24.4232;e=-6.6083;
x1=0; x2=4.1135e-06;
f=@(x)a-(a-b)*cos(c*(x-x1)) + d*e*sin(c*(x-x1))

f = function_handle with value:
    @(x)a-(a-b)*cos(c*(x-x1))+d*e*sin(c*(x-x1))

fplot(f, [x1 x2])

options = optimset('TolX',1e-8);
[xmin, min]=fminbnd(f, x1, x2, options)

xmin = 2.8422e-07

min = -996.4246

[Zhe Yu]

Hi Torsten, thank you very much!