I need to create a vector from a symbolic polynomial function,but when i use sym2poly command, matlab tells me it is not a polynomial, how should i do?

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sebastiano della gatta 2021-12-16

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https://ww2.mathworks.cn/matlabcentral/answers/1612445-i-need-to-create-a-vector-from-a-symbolic-polynomial-function-but-when-i-use-sym2poly-command-matla

评论： Star Strider 2021-12-17

f1=(497*c1^2 + 497*c2^2 + 497*c3^2 + 11*(c3 + c1*c2)*(c2 - c1*c3)*(c1^2 + c2^2 + c3^2 + 1) + 497)^2/(103684*(c1^2 + c2^2 + c3^2 + 1)^2) + (136*c1^2 + 136*c2^2 + 136*c3^2 + (11*(c2 - c1*c3)*(c1^2 - c2^2 - c3^2 + 1)*(c1^2 + c2^2 + c3^2 + 1))/2 + 136)^2/(103684*(c1^2 + c2^2 + c3^2 + 1)^2) - 1;

sym2poly(f1)

Error using sym/sym2poly (line 30)

Not a polynomial

why matlab tells me this?

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Answer 1

Star Strider 2021-12-16

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1612445-i-need-to-create-a-vector-from-a-symbolic-polynomial-function-but-when-i-use-sym2poly-command-matla#answer_856605

在 MATLAB Online 中打开

One option is to use the coeffs function —

syms c1 c2 c3

sympref('AbbreviateOutput',false);

f1=(497*c1^2 + 497*c2^2 + 497*c3^2 + 11*(c3 + c1*c2)*(c2 - c1*c3)*(c1^2 + c2^2 + c3^2 + 1) + 497)^2/(103684*(c1^2 + c2^2 + c3^2 + 1)^2) + (136*c1^2 + 136*c2^2 + 136*c3^2 + (11*(c2 - c1*c3)*(c1^2 - c2^2 - c3^2 + 1)*(c1^2 + c2^2 + c3^2 + 1))/2 + 136)^2/(103684*(c1^2 + c2^2 + c3^2 + 1)^2) - 1

f1 =

f1 = simplify(f1, 500)

f1 =

[f1n, f1d] = numden(f1)

f1n =

f1d =

414736

[f1n_cfs, f1n_trms] = coeffs(f1n) % Numerator Coefficients & Terms

f1n_cfs =

f1n_trms =

[f1n_cfs_all, trms_all] = coeffs(f1, 'All')

f1n_cfs_all(:,:,1) =

f1n_cfs_all(:,:,2) =

f1n_cfs_all(:,:,3) =

f1n_cfs_all(:,:,4) =

f1n_cfs_all(:,:,5) =

f1n_cfs_all(:,:,6) =

f1n_cfs_all(:,:,7) =

trms_all(:,:,1) =

trms_all(:,:,2) =

trms_all(:,:,3) =

trms_all(:,:,4) =

trms_all(:,:,5) =

trms_all(:,:,6) =

trms_all(:,:,7) =

That is as completely and thoroughly as it is possible to decompose this polynomial expression.

.

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sebastiano della gatta 2021-12-17

thank you for the answer but I don't want to completely decompose the polynomial, I just need the vector of this function because then I have to apply the grobner bases to arrive at my solution.