Generalized Cauchy–Schwarz inequality for real powers

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Let nn be a positive integer, let p≥2p\geq 2 be a real number, and let v,w∈R>0n\mathbf{v},\mathbf{w}\in\mathbb{R}_{>0}^n. For a vector, write vp\mathbf{v}^p for the entrywise pp-th power, and let ⟨⋅,⋅⟩\langle\cdot,\cdot\rangle and ∥⋅∥\|\cdot\| denote the Euclidean inner product and norm, respectively.

Generalized Cauchy–Schwarz conjecture. For every such pp, v\mathbf{v}, and w\mathbf{w},

∥vp∥∥wp∥−⟨vp,wp⟩≤∥v∥p∥w∥p−⟨v,w⟩p.\|\mathbf{v}^p\|\|\mathbf{w}^p\|-\langle\mathbf{v}^p,\mathbf{w}^p\rangle\leq \|\mathbf{v}\|^p\|\mathbf{w}\|^p-\langle\mathbf{v},\mathbf{w}\rangle^p.

The inequality is known for positive integer pp and fails for p∈(0,1)p\in(0,1) and p∈(1,2)p\in(1,2). Extensive numerical testing for non-integer p∈(2,10)p\in(2,10) and dimensions 2≤n≤102\leq n\leq 10 found no counterexamples, but the assertion for non-integer p≥2p\geq2 remains open; it is tight when v=w=e1\mathbf{v}=\mathbf{w}=\mathbf{e}_1.

References

Primary source

Nathaniel Johnston, Sarah Plosker, Charles Torrance and Luis M. B. Varona, “Generalizing the Cauchy-Schwarz inequality: Hadamard powers and tensor products”, arXiv:2507.10327 (2025).

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