I am learning calculus/real analysis with Apostol's Calculus (2nd Edition). I have a doubt about the grammer of this book. Apostol, everywhere, uses a supremum (or a least upper bound) and an infimum (or a greatest lower bound) for some set $S$.
How many suprema and infima can a set have that he has used determinents like a and an instead of the.
As far as my learning is concerned, I was sure about the fact that supremum and infimum are unique for a given set.