Peetre's inequality

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In mathematics, Peetre's inequality, named after Jaak Peetre, says that for any real number [math]\displaystyle{ t }[/math] and any vectors [math]\displaystyle{ x }[/math] and [math]\displaystyle{ y }[/math] in [math]\displaystyle{ \Reals^n, }[/math] the following inequality holds: [math]\displaystyle{ \left(\frac{1+|x|^2}{1+|y|^2}\right)^t ~\leq~ 2^{|t|} (1+|x-y|^2)^{|t|}. }[/math]

The inequality was proved by J. Peetre in 1959 and has founds applications in functional analysis and Sobolev spaces.

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