As I understand, you are trying to plot the equation attached in the figure, upon finding “f” as a function of “i”, but the function “symsum” is throwing an error.
The summation index in each of the equations must be declared as symbolic variables as stated in the documentation of the “symsum” function below- https://www.mathworks.com/help/symbolic/sym.symsum.html#btydjwt-k:~:text=%E2%80%94%20Summation%20index-,symbolic%20variable,-a%20%E2%80%94%20Lower.
This can be done using the “syms” command, which is used to create symbolic scalar variables and functions. For more details about “syms”, refer to this link - https://www.mathworks.com/help/symbolic/syms.html. Hence, in the code, before declaring the equation, include the following command
syms i j k l
The equation “f” is a polynomial with “i” as the variable, hence it can be declared as a symbolic function, so that it can be evaluated at different values of “i”. Refer to this documentation link which explains how to create and evaluate symbolic functions - https://www.mathworks.com/help/symbolic/syms.html#buoeaym-1:~:text=Create%20symbolic%20functions%20with%20one%20and%20two%20arguments.
Hope this helps!
