EN
We investigate weighted norm inequalities for the commutator of a fractional integral operator and multiplication by a function. In particular, we show that, for $μ,λ ∈ A_{p,q}$ and α/n + 1/q = 1/p, the norm $||[b,I_{α}]: L^{p}(μ^{p}) → L^{q}(λ^{q})||$ is equivalent to the norm of b in the weighted BMO space BMO(ν), where $ν = μλ^{-1}$. This work extends some of the results on this topic existing in the literature, and continues a line of investigation which was initiated by Bloom in 1985 and was recently developed further by the first author, Lacey, and Wick.