1991 USAMO Problems/Problem 3
Show that, for any fixed integer the sequence is eventually constant.
[The tower of exponents is defined by . Also means the remainder which results from dividing by .]
Suppose that the problem statement is false for some integer . Then there is a least , which we call , for which the statement is false.
Since all integers are equivalent mod 1, .
Note that for all integers , the sequence eventually becomes cyclic mod . Let be the period of this cycle. Since there are nonzero residues mod . . Since does not become constant mod , it follows the sequence of exponents of these terms, i.e., the sequence does not become constant mod . Then the problem statement is false for . Since , this is a contradiction. Therefore the problem statement is true.
Note that we may replace 2 with any other positive integer, and both the problem and this solution remain valid.
Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.
|1991 USAMO (Problems • Resources)|
|1 • 2 • 3 • 4 • 5|
|All USAMO Problems and Solutions|
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.