Operator Inequalities in $Φ$-Product Tensor Algebras: Invariance and Transform Sensitivity

Abstract

We study classical operator inequalities in $Φ$-product tensor algebras (a transformation $Φ$-based generalization of the $t$-product framework) for third-order tensors. Although these algebras are algebraically isomorphic under different unitary transforms $Φ$, we show that their quantitative behavior is not invariant. We prove that fundamental inequalities, including Golden--Thompson, Jensen, Klein, and Lieb, extend to the $Φ$-product setting with the same constants as in the matrix case. However, the associated defect -- the slack between the two sides of the inequality -- depends explicitly on transform-domain noncommutativity. In particular, we establish a sharp characterization of the defect in terms of slice-wise commutators, revealing that inequality tightness is governed by transform-induced noncommutativity. We further demonstrate strong transform sensitivity by constructing explicit tensor pairs for which the defect vanishes under one transform (e.g., discrete Fourier transform) but grows linearly with the tensor depth under another transform (e.g., discrete Cosine transform), yielding an $Ω(p)$ separation where $p$ is the matrix dimension of $Φ$. Moreover, we prove that no transform is universally optimal: for any pair of transforms, there exist tensors for which each is strictly better than the other. These results show that the choice of transform defines a coordinate system in which commutativity is measured, inducing a nontrivial geometry of inequality tightness. Consequently, optimal transform selection is inherently data-dependent and can be formulated as an optimization problem over the unitary group.