Algebraic Theory of Measurement Processes in Quantum Systems
  Here we deal in a pedagogical way with an approach to construct an algebraic structure for the Quantum Mechanical measurement processes from the concept of \emph{Measurement Symbol}. Such concept was conceived by Julian S. Schwinger and constitutes a fundamental piece in his variational formalism and its several applications. 
C. A. M. de Melo, B. M. Pimentel, J. A. Ramirez
quant-ph math-ph math.MP