  Building upon Hovey's work on Smith ideals for monoids, we develop a homotopy
theory of Smith ideals for general operads in a symmetric monoidal category.
For a sufficiently nice stable monoidal model category and an operad satisfying
a cofibrancy condition, we show that there is a Quillen equivalence between a
model structure on Smith ideals and a model structure on algebra maps induced
by the cokernel and the kernel. For symmetric spectra, this applies to the
commutative operad and all Sigma-cofibrant operads. For chain complexes over a
field of characteristic zero and the stable module category, this Quillen
equivalence holds for all operads. This paper ends with a comparison between
the semi-model category approach and the $\infty$-category approach to encoding
the homotopy theory of algebras over Sigma-cofibrant operads that are not
necessarily admissible.
