We solve a long-standing open problem in the theory of weighted inequalities concerning iterated Copson operators. We use a constructive approximation method based on a new discretization principle.
As a result, we characterize all weight functions w; v; u on (0,infinity) for which there exists a constant C such that the inequality (integral(infinity)(0)(integral(infinity)(t)(integral(infinity)(s)h(y)dy)(m) u(s)ds)(q/m)omega(t)dt)(1/q) = 1 because otherwise the inequality cannot hold for non-trivial weights, but otherwise p,q and m are unrestricted.