Does \ell^1 have metric-Markov-cotype-2?

The problem asks whether $\ell^1$ has metric Markov cotype $2$. Published work records this as an open question and notes that a positive answer would resolve a longstanding Lipschitz-extension problem attributed to Ball.

Known results

• Some closed subspaces of $\ell^1$ fail to have metric Markov cotype; this does not settle $\ell^1$ itself.
• Metric Markov cotype $2$ for $\ell^1$ would imply finiteness of $e(\ell_2,\ell_1)$.

September 10, 2026 community submission

A submitted proof argues that real and complex $L_1$ spaces have metric Markov cotype $2$, with explicit universal constants. The argument is unverified and has no independently checkable supporting source.

Current status (as of September 2026): The question remains open in the published record, while a community submission claims a positive answer that is unverified.