Tree Coordinates and Range Martingales for Positive Operator-Valued Measures

Abstract

Positive operator-valued measures on a tree admit intrinsic local coordinates coming from the way each cylinder value splits into its children. We show that these local splittings, taken on the range spaces of the cylinder values, recover the measure and at the same time build an intrinsic direct limit dilation whose cylinder projections yield the minimal Naimark dilation. In these coordinates, the commutant of the dilation becomes a martingale calculus on the range spaces. This gives local descriptions of extremality and domination, and it also yields a bounded change-of-measure transform that updates the tree coordinates in a natural way. For self-adjoint range martingales we obtain a quadratic variation formula from the range space isometries, and the associated local variance terms detect the projection-valued case.