In axiomatic quantum theory, S -matrix is considered as a functional of a function g : M → → [ 0 , 1 ] defined on the Minkowski space M . This function characterizes the intensity of the interaction in different space-time regions: the value g ( x ) = 0 at a point x corresponds to the absence of interaction in x , g ( x ) = 1 corresponds to the most intense interaction, and values between 0 and 1 correspond to incomplete interaction at x . For two points x , y ∈ ∈ M , the notation x ≤ ≤ y means that x causally precedes y .