Separability and decomposition in mechanism design with transfers

In private values quasi-linear environment, we consider problems where allocation decisions along multiple components have to be made. Every agent has additively separable valuation over the components. We show that every unanimous and dominant strategy implementable allocation rule in this problem is a component-wise weighted utilitarian rule, which assigns non-negative weight vectors to agents in each component and chooses an alternative in each component by maximizing the weighted sum of valuations in that component. A corollary of our result is that every unanimous and dominant strategy implementable allocation rule can be almost decomposed (modulo tie-breaking) into dominant strategy implementable allocation rules along each component.