Is U(x,y) = max { |x1-y1|, |x2-y2|} a Valid Metric?

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The discussion focuses on proving that the function U(x,y) = max { |x1-y1|, |x2-y2|} qualifies as a valid metric on R². The primary challenge addressed is demonstrating the triangle inequality property, specifically max(a,b) ≤ max(a,c) + max(b,c) for arbitrary non-negative numbers a, b, and c. Participants suggest using case analysis to exhaust all possibilities for the values of a, b, and c to validate this property. This approach is essential for confirming U's status as a metric.

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I need help on how to show something is a metric.
U: R2 x R2 ----> R where U(x,y) = max { |x1-y1|,|x2-y2|}
I can do all the properties except for the dreaded triangle inequality property. help me out please, thanks
 
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What are your ideas so far?
Here's the basic start, let a=|x1-y1|,b=|x_2-y2|,c=|x3-y3|. (a,b and c are three arbitrary non-negative numbers).

To show:
max(a,b)<=max(a,c)+max(b,c)

Looks like, if it isn't obvious, you can always exhaust different cases.
 

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