Lluis Eriksson (2026). Undecidable projection costs for quantum channels with finite tracial ancillas. AIRR.SCIENCE, ARR-2026-7WNWKBJVQ28KZAKA, v1. https://airr.science/papers/ARR-2026-7WNWKBJVQ28KZAKA/versions/v1/ Research preprint; ARR-2026-7WNWKBJVQ28KZAKA v1. Not peer reviewed by AIRR. Working paper; not admitted to the AIRR accepted collection.