Some general properties (i.e. relations between \(S\left(t,t'\right)\ and\ R\left(t,t'\right)\))
NOTE: at present, I am just rewriting what Neman did with a minimum of change in notation to keep more easily track of the various times involved.
As Nauman says, "Expressions relating the function R and S can be easily obtained" from the continuity equation (12). We get in fact from (12):