Example 4 (15th Asia Pacific Mathematical Olympiad) Let be the largest prime less than . If , and is a composite number, prove:
(1) If , then does not divide !.
(2) If , then divides !.
Solution
Prove (1) When ,
Since , then , so, , hence .
(2) When ,
Since is a composite number, let , if .
( i ) , then .
Thus, .
So .
(ii) , then ,
Since , then ,
Thus . Hence . So, .
If , since , assume is not a prime, then , since , then , thus falls into the case of ,
Assume is a prime, then ,
Since is the largest prime less than , then ,
Thus . So, .
In summary, when , .
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