Error in defining imaginary symbolic variable in poly2sym

>> 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?

[madhan ravi]

Could you explicitly write how your expected result should look like?

Answer 1

[Aniket]

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!