- #1

- 1,105

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Does this hold for a summation of say Sigma (|xn+yn|) <= sigma|xn| + sigma|yn| for n=0 to infinity?

would this also work for sup|x+y| ??

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- Thread starter gravenewworld
- Start date

- #1

- 1,105

- 25

Does this hold for a summation of say Sigma (|xn+yn|) <= sigma|xn| + sigma|yn| for n=0 to infinity?

would this also work for sup|x+y| ??

- #2

LeonhardEuler

Gold Member

- 859

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Yes:gravenewworld said:I know that the triangle inequality is lx+y|<= |x| +|y|

Does this hold for a summation of say Sigma (|xn+yn|) <= sigma|xn| + sigma|yn| for n=0 to infinity?

[tex]\sum_{n=0}^{N}{|x_n+y_n|}[/tex]

[tex]\leq |x_0| + |y_0| + \sum_{n=1}^{N}{|x_{n}+y_n|}[/tex]

[tex]\leq |x_0| + |y_0| + |x_1| + |y_1| + \sum_{n=2}^{N}{|x_{n}+y_n|}[/tex]

...

[tex]\leq \sum_{n=0}^{N}{|x_n|} +\sum_{n=0}^{N}{|y_n|}[/tex]

Last edited:

- #3

AKG

Science Advisor

Homework Helper

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Suppose all the series in question do in fact converge. Then suppose the desired inequality is not true, then we'd have:

[tex]\sum _{n = 0} ^{\infty} |x_n + y_n| > \sum _{n=0} ^{\infty} |x_n| + \sum _{n=0} ^{\infty} |y_n|[/tex]

[tex]\lim _{N \to \infty} \left ( \sum _{n = 0} ^N |x_n + y_n|\right ) > \lim _{N \to \infty} \left ( \sum _{n = 0} ^N |x_n|\right ) + \lim _{N \to \infty} \left ( \sum _{n = 0} ^N |y_n|\right )[/tex]

[tex]\lim _{N \to \infty} \left ( \sum _{n = 0} ^N |x_n + y_n|\right ) > \lim _{N \to \infty} \left ( \sum _{n = 0} ^N |x_n| + \sum _{n = 0} ^N |y_n|\right )[/tex]

thus there is some N such that:

[tex]\sum _{n = 0} ^N |x_n + y_n| > \sum _{n = 0} ^N |x_n|\right + \sum _{n = 0} ^N |y_n|[/tex]

which LeonhardEuler has proven false.

- #4

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- 25

alright thanks guys!

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