trying to find all critical points where gradient f = 0 but am getting an error.

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Peter Pyrka 2019-11-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/492492-trying-to-find-all-critical-points-where-gradient-f-0-but-am-getting-an-error

评论： Walter Roberson 2019-11-22

This is what my code looks like right now. I am getting an error with the 'Real' part of my code with it saying that it is invalid.

%Storing the function f and g

syms w x y z;
f(w,x,y,z) = 2.0*w*x - 4.0*y - 5.0*w*y + 4.0*y*z + w^2 + 4.0*z^2;
g(w,x,y,z) = 0.4723*w*x + 0.6455*w*y + 0.1187*x*y + 0.8042*w^2 + 0.08831*x^2 + 0.2568*y^2 + 0.06482*z^2;
%Getting the derivatives of both functions
Dfw = diff(f,w);
Dfx = diff(f,x);
Dfy = diff(f,y);
Dfz = diff(f,z);
Dgw = diff(g,w);
Dgx = diff(g,x);
Dgy = diff(g,y);
Dgz = diff(g,z);
%Finding All Critical points by finding where gradient f is zero
[wSols, xSols, ySols, zSols] = solve([Dfw == 0, Dfx == 0, Dfy == 0, Dfz == 0], [w,x,y,z],'Real', 'true')

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采纳的回答

Walter Roberson 2019-11-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/492492-trying-to-find-all-critical-points-where-gradient-f-0-but-am-getting-an-error#answer_402710

Use 'Real', true instead of 'Real', 'true'

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Peter Pyrka 2019-11-22

Okay but just to clarify, my output should be correct then with having only 1 solution for each and not more since it is strictly linear?

Walter Roberson 2019-11-22

There would have to be a subexpression with degree 3 or higher for the partial derivatives to be non-linear, so having only one solution is expected for these.

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