# Homework Help: Continuity of the the norm

1. Feb 23, 2010

### gtfitzpatrick

1. The problem statement, all variables and given/known data
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Prove the continuity of the norm; ie show that in any n.l.s. N if xn $$\rightarrow$$ x then $$\left|\left|x_n\left|\left|$$ $$\rightarrow$$ $$\left|\left|x\left|\left|$$

3. The attempt at a solution

i dont know where to start this
from the definition of convergence xn $$\rightarrow$$ x as n$$\rightarrow$$ $$\infty$$ if $$\left|\left|x_n - X\left|\left|$$ $$\rightarrow$$ 0 as n$$\rightarrow$$ $$\infty$$

do i use this to prove it or am i barking up the wrong tree?

2. Feb 23, 2010

### Dick

Sure you use the definition of convergence. Can you prove the 'reverse triangle inequality'? abs(||a||-||b||)<=||a-b||.

3. Feb 23, 2010

### gtfitzpatrick

let |a| ≥ |b|,
However, we can write a as a - b + b or a = (a - b) + b

Then |a| = |a - b + b|. But, |a - b + b| $$\leq$$ |a - b| + |b| by the Triangle Inequality, and so we have that |a| $$\leq$$ |a - b| + |b| . Now, subtract |b| from both sides. This gives us: |a|-|b| $$\leq$$ |a - b|

right?

4. Feb 23, 2010

### Dick

Sure. That's it. Makes your proof easy, right?

5. Feb 23, 2010

### gtfitzpatrick

thanks a million, i guess i'm just not looking at them right, but now i see