For an integer pair , let . If for some positive integer the following statement holds:
If integers satisfy that divides , then divides .
then the pair is called "-sufficient". If there exist infinitely many positive integers such that is "-sufficient", then the pair is called "very sufficient". Question: does there exist a pair such that is "1110-sufficient", but not "very sufficient"?
Solution
The answer is no! We will prove below that if is "1110-sufficient", then for any , is -sufficient.
(1) If is 1110-sufficient, then is 37-sufficient.
Suppose is not 37-sufficient, then there exist such that but is not a multiple of 37. By the Chinese Remainder Theorem, there exist integers such that both and are multiples of 30, and , . Thus we have and , that is, . Hence 1110 does not divide , giving a contradiction.
(2) If is 37-sufficient, then .
By contradiction, suppose is not a multiple of 37. First, if is also not a multiple of 37, note that , and . Let the sets
where the residues here are all taken between 0 and 36. Then both and have 19 elements, so there must be overlap between them; that is, there exist integers such that .
Since 37 does not divide , and are not both 0, so we may take or , such that is not a multiple of 37. Take an integer such that , then
and since is not a multiple of 37, is not a multiple of 37, that is, and have different residues mod 37. However we have , a contradiction.
If is a multiple of 37, then is a multiple of 37, also a contradiction, so (2) is proved.
(3) For any positive integer , is -sufficient.
By (2), . Clearly cannot be a multiple of 37, otherwise , a contradiction. For any integers , if , then note that is not a multiple of 37, hence , which completes the proof.