Determine all integers such that for all , divides the binomial coefficient .
Solution
Solution: The integers satisfying the problem's condition are all primes.
First we check that all primes satisfy the condition of the problem. That is, if is a prime, then for all , , divides the binomial coefficient . When this holds. For an odd prime , take and consider
Since and is odd, every factor in the above expression is nonzero. If , then divides , but , so . This shows that and are coprime. Therefore divides the binomial coefficient .
Next, we prove that no composite number satisfies the property of the problem. Consider two cases:
(1) If , take . Then , but cannot be divided by .
(2) If is odd, then there exists an odd prime and an integer such that . Take , then from we get . However
is not an integer, because divides the denominator but does not divide the numerator.