In analysis and analytic number theory, one sometimes obtains estimates for expressions involving several variables or parameters subject to various constraints concerning their respective sizes. If these estimates have to be carried along and transformed in substantial ways, or if many of them involving different constraints must be combined at some point, it may be useful (if it is possible) to incorporate the constraints inside the estimates by adding terms which make the estimate trivial if the constraints are not satisfied, and reduce to the useful estimate otherwise.
This is similar in spirit to the use of a characteristic function of a set to represent a summation or integration condition, as described in Getting rid of nasty cutoffs for example.
Basic calculus or even simple combinatorics and sums.
The idea of the trick is best explained with concrete elementary examples.
Consider the inequality
This is only valid if . However, if we write
the inequality is valid for , and is not asymptotically worse than the previous one when gets large.
Suppose you have a function depending on an integer and a prime number , and that you know the upper bound
for all , and
if does not divide . If these estimates are used extensively later on, with different values of and , sorting out which of the two applies may become a bookkeeping nightmare. Writing
where is the GCD of and encapsulates both statements neatly and is much easier to carry around and combine with other estimates.