Using Pearson Correlation Coefficient in computing user/item similarity I'm researching for an algorithm for item-based/user-based collaborative filtering and I've come to this site.

It uses Pearson correlation coefficient to compute similarity between users and when I read this line:


  A strength of this formula is that it takes into account average
  ratings for each user. So if user a rates everything a 5 and user u
  rates everything a 1, then they still have a similarity rating of 1.


I wonder why it would give a similarity of 1 because using the said equation which is the Pearson and using the given (user a, 5 and user u, 1), it would give a zero for the numerator and denominator.

So what is the reason he said that the similarity of user a and user u with a rating of 5 and 1, respectively is equal to one? Is zero over zero equals to one?
Pearson's correlation is undefined when the variance of one of the random variables is 0, I would look for a more credible source to read from.