I am writing my thesis and my advisor made a correction to the sentence below:
Transitional Rule commonly denoted by $\phi$ is defined by the map $\Sigma^n \rightarrow \Sigma$.
He has changed the sentence by changing the definite article the to the indefinite article a as below:
Transitional Rule commonly denoted by $\phi$ is defined by a map $\Sigma^n \rightarrow \Sigma$.
Which of the sentences above are correct providing that $\Sigma$ is a set that is defined previously and n is the length of a string?
I have already asked this question in English Stack Exchange and I could not get any answer. They have said this a technical question. Thanks in advance.