Almost-Orthogonality in Lp Spaces: A Case Study with Grok
Carbery proposed the following sharpened form of triangle inequality for many functions: for any p≥2 and any finite sequence (fj)j⊂Lp we have ∑jfjp ≤ (supj∑kαjkc)1/p′(∑j∥fj∥pp)1/p, where c=2, 1/p+1/p′=1, and αjk=∥fj∥p∥fk∥p∥fjfk∥p/2. In the first part of this paper we construct a counterexample showing that this inequality fails for every p>2. We then prove that if an estimate of the above form holds, the exponent must satisfy c≤p′. Finally, at the critical exponent c=p′, we establish the inequality for all integer values p≥2. In the second part of the paper we obtain a sharp three-function bound ∑j=13fjp ≤ (1+2Γc(p))1/p′(∑j=13∥fj∥pp)1/p, where p≥3, c(p)=(p−2)ln(3)+2ln(2)2ln(2) and Γ=Γ(f1,f2,f3)∈[0,1] quantifies the degree of orthogonality among f1,f2,f3. The exponent c(p) is optimal, and improves upon the power r(p)=5p−46 obtained previously by Carlen, Frank, and Lieb. Some intermediate lemmas and inequalities appearing in this work were explored with the assistance of the large language model Grok.