Do you know the
definitions for these things?
"Lower bound". x is a lower bound for set Y if and only if, whenever y is in Y,
.
Every number in that set is clearly positive so every non-negative number (0 and negatives) is less than any number in that set.
No positive number can be a lower bound. Let x> 0. Then 1/x is also a positive number and there exist an integer n> 1/x (Archimedian property). Then 1/n< x.
"Greatest lower bound". x is a greatest lower bound for the set Y if it is a lower bound for Y and every other lower bound for Y is less than x. It literaly is the "largest" of all lower bounds.
Here, the set of lower bounds is the set of nonnegative numbers: [tex]\{x | x\le 0\}. 0 is in that set and is larger than any other number in the set. 0 is the greatest lower bound.