00:01
Here we are required to first show that open info from a to b is not complete in the real line r.
00:19
Okay, to make it clear, we need to equip the absolute value on the real line, i mean the distance between two points in our real line.
00:31
It's just the absolute value of their difference.
00:35
Okay, so we want to prove this open info is not complete with respect to the absolute value in real line.
00:44
Okay, it's very easy because we say a subspace is a complete subspace if and only if for any cauchy sequence, in, let's say a, b, let's say r, n, we know we always have r converges to, rn converges to some r in this subspace.
01:24
So here we only need to show it is incomplete, we only need to find a cauchy sequence that is not convergent or that converges, that will converge to something not in this interval.
01:38
Okay, it's very easy, it's easy because let's just define n, let's just define rn to be just equal to a plus one over n.
01:48
Then, just by some fundamental discussion in calculus, we know the limit where n approaches positive phase rn will be equal to a.
02:02
This, or equivalent, rn converges to a under this absolute value.
02:10
However, we know a is of course not contained in our open info, that means this guy, ab, is not convergent, is not, is incomplete.
02:32
Okay, now let's just consider the closed info, closed info from a to b...