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I am trying to find out when equality holds in Minkowski's inequality for $L^{\infty}$ (i.e. a necessary and sufficient condition for equality). I did a search and there was a discussion for the case where $1 but not when $p=\infty$ so I am hoping to get some ideas or for someone to point me to a source where this is discussed.

I will list a couple of observations I made while working this out (though I'm not sure whether I'm right with these):

  1. If $\mu(\{x:|f(x)|\geq\|f+g\|_{\infty}-\|g\|_{\infty}\})=0$ (or with $f$ and $g$ interchanged), then I have the reverse inequality.

  2. If I pick $a,b$ such that $\|f\|_{\infty}\leq a<\|f\|_{\infty}+\varepsilon$, $\|g\|_{\infty}\leq b<\|g\|_{\infty}+\varepsilon$, $\mu(\{x:|f(x)|>a\})=0$, $\mu(\{x:|g(x)|>b\})=0$, and for all $c I have $\mu(\{x:|f(x)+g(x)|>c\})>0$, then I also have the reverse inequality.

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    Since the $L^\infty$ norm is defined via supremum, it would be helpful to think when you have $\sup(A+B) = \sup A + \sup B$.2016-05-12

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There is no simple condition for equality here. For example if $f=\chi_{(0,3)},g=\chi_{(1,2)}$ then $||f+g||_\infty =||f||_\infty +||g||_\infty$.But f and g are not multiples of each other. I hope this example convinces you that one cannot write down simple N & S conditions for equality in Minkowski's inequality.