Bug fminbnd not working

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Stephen Wilkerson
Stephen Wilkerson 2019-1-20
编辑： Torsten 2022-7-17
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?
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Stephen Wilkerson
Stephen Wilkerson 2019-1-20
Not where the minimum is! look at the plot
Stephen Wilkerson
Stephen Wilkerson 2019-1-20
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

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Walter Roberson
Walter Roberson 2019-1-20
No bug. fminbnd is a local minimizer, not a global minimizer.
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Torsten
Torsten 2022-7-17
编辑：Torsten 2022-7-17
Ran in:
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
Zhe Yu 2022-7-17
Hi Torsten, thank you very much!

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