surface plot with a matrix

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konoha 2016-4-2

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https://ww2.mathworks.cn/matlabcentral/answers/276784-surface-plot-with-a-matrix

评论： Azzi Abdelmalek 2016-4-2

I am trying to graph a surface with a diagonal matrix, the equation I am trying graph is f = x^TDx, x is a 2 by 1 vector and D is a 2 by 2 matrix.

Here is what have so far, but I keep getting error.

x = linspace(-10,10);
y = linspace(-10,10);
[X,Y] = meshgrid(x,y);
D = [2 1; 1 2];
f = @(x,y) [x,y]*D*[x,y].'; % [x,y] is 1 by 2 
contour (X,Y,f(X,Y))

I know that would give me an erros since the dimension of [x,y] and D not macth, I know that if D is a identity matrix, I could use eye(2*numel(x)) to create a 200x200 matrix to match [x,y].

I don't know how to do it with a random and a given diagonal symmetric matrix. Can someone show me how do it? Thanks.