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We have the standard $1$ and $2$ norms $$ ||x||_1 := \sum_{i = 1}^n |x_i|, \qquad ||x||_2 := \sqrt{\sum_{i = 1}^n x_i^2}, $$ where $n \in \mathbb{N}$ and $x = (x_i)_{i = 1}^n \in \mathbb{Z}^n$. On the math.stackexchange page there are proofs of the inequalities $$ ||x||_2 \leqslant ||x||_1 \leqslant \sqrt{n}||x||_2. $$

I am interested in the weighted $1$ and $2$ norms $$ ||x||_1^w := \sum_{i = 1}^n w_i|x_i|, \qquad ||x||_2^w := \sqrt{\sum_{i = 1}^n w_ix_i^2}, $$ where $w = (w_i)_{i = 1}^n \in \mathbb{N}^n$.

Could you give me any reference in the literature to cite some results about these weighted norms in my scientific paper? In particular, I would like to know for them a proper version of the above inequalities. ''Proper'' means that the inequalities should be sufficiently optimal.

I guess that the theory of the norms $||\cdot||_1^w$ and $||\cdot||_2^w$ is quite classical to be already discussed somewhere. It is not correct and too long if I develop this theory from scratch in my paper. I cannot be the first author in the given research direction.

If it is difficult to find a reference in the literature and the result seems folklore, I will appreciate your help in formulating and proving the inequalities in question. Thanks in advance for any comment!

Note that $||x||_1^w = ||Wx||_1$ and $||x||_2^{w^2} = ||Wx||_2$, where $W := \mathrm{diag}(w_i)_{i = 1}^n$ and $w^2 := (w_i^2)_{i = 1}^n$. Consequently, $$ ||x||_2^{w^2} \leqslant ||x||_1^w \leqslant \sqrt{n}||x||_2^{w^2}. $$ Nevertheless, you may be able to propose more optimal inequalities.

  • Hint: a norm is completely determined by the unit disc. The unit disc, $D_1$ say, of the 1 norm is a "generalised lozenge" that fits within the Euclidean disc $D_2$ say that is the unit disc of the 2 norm and this explains why $|x|_2 \le |x|_1$. If you scale $D_1$ by a suitable factor, you will find the explanation why $|x|_1 \le \sqrt{n}|x|_2$. Try a similar analysis of your weighted norms. (It will make life simpler to consider $n = 2$ and maybe $n = 3$ first. – Rob Arthan Jan 16 '25 at 21:22

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You can look at pages 74-95 of Mitrinovic's Analytic Inequalities book from Springer for a treatment of means and weighted means. His sequence $q_i$ is your $w_i:$

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Theorem 1 on page 79 on ratios of means might help:

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