Carbery's inequality in the Schatten--von Neumann classes
Carbery posed a question of sharpened triangle inequalities for families of operators in the Schatten--von Neumann classes $S_p$, $p\geq 2$. He established a weaker form of the desired estimate for even integer values of $p$. In the commutative setting the corresponding sharp inequality (with optimal exponent $p'=\frac{p}{p-1}$) was recently obtained for all integer $p\geq 2$. In the present work we resolve Carbery's question completely in the non-commutative setting: we prove the sharp inequality $$ \Big\|\sum_{j} T_{j}\Big\|_{S_{p}}\leq \|(\alpha_{ij}^{p'})\|^{1/p'}_{\ell_{2}\to \ell_{2}} \Big( \sum_{j} \|T_{j}\|^{p}_{S_{p}}\Big)^{1/p} $$ for all $p\geq 2$ and all countable sequences of operators $(T_{j}) \subset S_{p}$, where $\alpha_{ij}$ are almost orthogonality coefficients. The proof is based on a block-operator reduction and a complex interpolation of the polar parts of the blocks.