For IMO Problem 1, a key lemma reduces cases with n > 3, such as n = 5, to smaller cases n = 4 and n = 3.

“the all the possibilities for different cases like for different n uh for the any any positive integer that greater than a greater or equal to three and so it requires you to find a key lemma which is like uh for any n greater than three like let's say n equal to five you can”

“reduce the problem to n equal to 4 and reduce the problem to n equal to 3 and”