The two probabilities above are clearly different (also in form) and we do not use further subscripts to say that, since it is clear from the argument (two diverse variables)
- "the fraction of material entering the reactor (control volume) at time \(t\) which will remain in the reactor for a duration greater than \(t'\) " (citation from Neuman paper) can be then expressed as an integral over the first of the above probabilities: