- #1
futurebird
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I'm studying the proof that the l2 metric space is complete and I just don't get something about this proof.
What I know:
Suppose that for e > o there exists n0 such that:
||fn - fm||2 < e when m, n >= n0.
I don't understand how to think about ||fn - fm||2.
Suppose fn = .1, .01, .001, ...
Suppose fm = .2, .02, .002, ...
How would I find ||fn - fm||2 computationally?
Do I do:
||fn - fm||2 = (|(.12 + .012, .0012 + ...) - (.22 + .022 + .0022 + ...)|)0.5
||fn - fm||2 = (|(.01 + .001, .0001 + ...) - (.04+ .004 + .0004 + ...)|)0.5
||fn - fm||2 = (|(.01 + .001, .0001 + ...) - (.04 + .004 + .0004 + ...)|)0.5
||fn - fm||2 = ( |1/90 - 4/90|)0.5
||fn - fm||2 = (3/90)0.5
or
[(.1 - .2)2 + (.01-.02)2, + ...]0.5 ?
I think it's the first way... but for some reason I'm confused. Deeply.
What I know:
- l2 is the metric space whose elements are sequences that converge when you square each term and sum them. In other words, a sequence, f is in l2 if SUM( |f(k)|2) < infinity.
- The norm on this metric space is: (SUM( |f(k)|2))0.5 = || f ||2
- A sequence in the l2 space is a sequence of sequences (confusing isn't it?)
- To show that the space is complete I need to show that every cauchy sequence in l2 converges to a sequence in l2.
Suppose that for e > o there exists n0 such that:
||fn - fm||2 < e when m, n >= n0.
I don't understand how to think about ||fn - fm||2.
Suppose fn = .1, .01, .001, ...
Suppose fm = .2, .02, .002, ...
How would I find ||fn - fm||2 computationally?
Do I do:
||fn - fm||2 = (|(.12 + .012, .0012 + ...) - (.22 + .022 + .0022 + ...)|)0.5
||fn - fm||2 = (|(.01 + .001, .0001 + ...) - (.04+ .004 + .0004 + ...)|)0.5
||fn - fm||2 = (|(.01 + .001, .0001 + ...) - (.04 + .004 + .0004 + ...)|)0.5
||fn - fm||2 = ( |1/90 - 4/90|)0.5
||fn - fm||2 = (3/90)0.5
or
[(.1 - .2)2 + (.01-.02)2, + ...]0.5 ?
I think it's the first way... but for some reason I'm confused. Deeply.