Object ideals and idempotent completeness

Abstract

We construct an idempotent complete Frobenius exact category with an idempotent complete stable category in which a complete ideal cotorsion pair of object ideals induces a cotorsion pair of objects that fails to be complete. This gives a negative answer to the question of Fu, Guil Asensio, Herzog and Torrecillas under idempotent completeness. The construction uses a lemma on dense triangulated subcategories and an example of Bondarko and Sosnilo in which a weight structure fails to extend to the idempotent completion. The induced cotorsion pair is hereditary, and every object admits an ordinary precover from its left class and an ordinary preenvelope into its right class. We characterize which objects admit special precovers and which admit special preenvelopes.