Exact Tensorization Formulas for Jensen-Shannon Divergence Contraction: The Z-Channel and Binary Erasure Channel

Closed-form formulas for JSD contraction coefficients of tensor-product channels.

Key results:
- Edge Reduction Theorem with full proof
- Z-channel: coefficient depends on support set asymmetry, not Hamming distance
- Chernoff–JSD correspondence for boundary-optimizer channels

Note (March 2026): By the general identity η_f(W) = η_χ²(W) for all f-divergences (Ordentlich-Polyanskiy; Polyanskiy-Wu Theorem 33.6), all formulas in this work simultaneously give η_χ² for the same tensor-product channels. The results are not JSD-specific.