Positivity domains for pairs of triples of spin observables
  Positivity restrains the allowed domains for pairs or triples of spin observables in polarised reactions. Various domain shapes in ${1\over2}+{1\over2}\to{1\over2}+{1\over2}$ reactions are displayed. Some methods to determine these domains are mentioned and a new one based on the anticommutation between two observables is presented. 
X. Artru (IPNL), J.-M. Richard (LPSC), J. Soffer