The Classification of Decoherence Functionals: An Analogue of Gleason's   Theorem
  Gell-Mann and Hartle have proposed a significant generalisation of quantum theory with a scheme whose basic ingredients are `histories' and decoherence functionals. Within this scheme it is natural to identify the space $\UP$ of propositions about histories with an orthoalgebra or lattice. This raises the important problem of classifying the decoherence functionals in the case where $\UP$ is the lattice of projectors $\PV$ in some Hilbert space $\V$; in effect we seek the history analogue of Gleason's famous theorem in standard quantum theory. In the present paper we present the solution to this problem for the case where $\V$ is finite-dimensional. In particular, we show that every decoherence functional $d(\a,\b)$, $\a,\b\in\PV$ can be written in the form $d(\a,\b)=\tr_{\V\otimes\V}(\a\otimes\b X)$ for some operator $X$ on the tensor product space $\V\otimes\V$. 
Chris Isham, Noah Linden & Stefan Schreckenberg