Error in defining imaginary symbolic variable in poly2sym

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Arup
Arup 2020-2-25
回答： Aniket 2025-1-27
>> syms s;
FN1= [6,-4,-4,3,0];
FN2 = poly2sym(FN1,-1i*s);
Error using sym/poly2sym (line 27)
Second argument must be a symbolic variable.
>>
How to solve the problem?

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回答（1 个）

Aniket
Aniket 2025-1-27
Hi @Arup,
The error you are encountering is because the second argument of the "poly2sym" function must be a symbolic variable. The issue can be resolved by updating the code as mentioned below:
syms s;
FN1 = [6, -4, -4, 3, 0];
FN2 = poly2sym(FN1, s); % Use 's' as the symbolic variable
If you want to evaluate or manipulate the polynomial with "-1i*s", you can do that after creating the polynomial. For example:
FN2 = poly2sym(FN1, s); % Create the polynomial with 's'
FN2_at_neg_1i_s = subs(FN2, s, -1i*s); % Substitute '-1i*s' into the polynomial
This way, you first create the polynomial using "s" as the symbolic variable and then substitute "-1i*s" into the polynomial.
You can find more information regarding "poly2sym" function in this documentation: https://www.mathworks.com/help/symbolic/sym.poly2sym.html
Hope this helps!

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