What's wrong with my distribution function?

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Shing Ho Lin
Shing Ho Lin 2022-5-21
评论： VBBV 2022-5-21
I'm trying to draw a chi-square distribution in XYZ space. Chi-square distribution in Geogebra is supposed to be (with different index k):
That means, it is supposed to be a central-symmetric, volcano-shaped figure.
Here comes the code:
X = -100 : 1 : 100;
Y = -100 : 1 : 100;
k = 4;
G = gamma(k/2);
[ XX, YY ] = meshgrid( X, Y );
A = ( XX ).^2 + ( YY ).^2;
A = sqrt (A);
Z = (A^(k/2 - 1) * exp(-A / 2)) / ( 2^(k/2) * G );
figure, mesh(X, Y, Z);
Very unfortunately, the figure I generated is:
What exactly is wrong with my code?
  1 个评论
Shing Ho Lin
Shing Ho Lin 2022-5-21
It is most probably a silly question, but I just get stuck into this. Very much appreciated for any possible answer:)

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回答（1 个）

VBBV
VBBV 2022-5-21
Ran in:
X = -100 : 1 : 100;
Y = -100 : 1 : 100;
k = 4;
G = gamma(k/2)
G = 1
[ XX, YY ] = meshgrid( X, Y );
A = ( XX ).^2 + ( YY ).^2;
A = sqrt (A);
Z = (A.^(k/2-1).*exp(-A / 2))./(2^(k/2)* G); % use element wise operator
mesh(X, Y, Z);
axis([-20 20 -100 100 0 0.2])

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