Let's talk for a minute about a notation you have likely seen before. It is called the "Sigma" notation because that's the name of the Greek letter \(\Sigma\text<.>\) The notation involves an indexing variable which runs between a lower limit and an upper limit. The lower and upper limits are required to be integers\footnote. If the indexing variable is \(n\text\) the lower limit is \(L\text\) the upper limit is \(U\) and the general term is \(b_n\text\) the summation looks like \(\displaystyle\sum_^U b_n\text<.>\) What this means is to add together all the values of \(b_n\) starting with \(n=L\) and ending with \(n=U\text<.>\)
The summand, as you can see, is usually a function of the indexing variable; otherwise, the summand would not change from term to term. There may be other variables, for example \(\displaystyle\sum_^6 k x\) evaluates to \(3x + 4x + 5x + 6x\text\) which is equal to \(18x\text<.>\) Note that this other variable \(x\) persists when the sum is evaluated. It is a free variable. On the other hand, the index of summation, \(k\) in this case, is a bound variable. It runs over a set of values (in this case 3 to 6) and does not appear in the final value.
In the sum \(\displaystyle\sum_^n \frac\text\) which of the variables \(k\text\) \(n\) and \(C\) are free and which are bound?