In math and (especially) physics it is common to express "proportionality" laws usually with a symbol that looks like LaTeX \propto. So for instance the activity A of a radioactive sample is written A \propto e^(-t/T) where t is time and T is the mean lifetime of a single particle. For convenience \propto is often transliterated as ~ when typing. So that becomes

p_catch ~ n

p_spread ~ n

p_relay = p_spread * p_catch ~ n^2.