One - Tradução em português

I was reading a book and this sentence attracted my attention:

"(...) The error results from the fact that the proof does not work for T(2), since T(1) is not necessarily canceled by c. One should be very suspicious when a parameter (c in this case) that appears in the expression does not appear in the final solution."

What's a good Portuguese equivalent to that "one"?

Thank you in advance.