Invalid indexing when simplifying a symbolic expression

I am trying to somplfy an expression using the script below.

clc,clear
syms k4f k3f k3r k2f k2r C E0 S
eqn = k4f*(E0 - E0*(k3r+k4f) / (((k3f*k2f*C*S)/k2r)+(k3r*k4f)*(1+k2f*C/k2r)) - E0*k2f*C(k3r+k4f) / k2r*(((k3f*k2f*C*S)/k2r)+(k3r*k4f)*(1+k2f*C/k2r)));
simplfy(eqn)

When trying to simplfy "eqn", I get the following error messages...

Error using sym/subsindex (line 814)

Invalid indexing or function definition. Indexing

must follow MATLAB indexing. Function arguments

must be symbolic variables, and function body

must be sym expression.

Error in sym/subsref (line 859)

R_tilde =

builtin('subsref',L_tilde,Idx);

Error in Simplify_Eqn (line 3)

eqn = k4f*(E0 - E0*(k3r+k4f) /

(((k3f*k2f*C*S)/k2r)+(k3r*k4f)*(1+k2f*C/k2r)) -

E0*k2f*C(k3r+k4f) /

k2r*(((k3f*k2f*C*S)/k2r)+(k3r*k4f)*(1+k2f*C/k2r)));

>>

Any help is appreciated!

[madhan ravi]

C you’re missing an operator here (k3r+k4f)

Answer 1

[Fabio Freschi]

C is used as a function

C(k3r+k4f)