Let p be a prime larger than 7. What is the remainder when p! is divided by p+1?
I tried plugging in the next prime (11), which doesn't help with such big numbers. Then I tried writing p!p+1=p(p−1)(p−2).../(p+1). Dividing each of p, (p−1) etc individually by (p+1), I always get (p+1) as a remainder, and multiplying all those remainders together and dividing again by p+1 will give 0, which I'm not sure is right. How can I solve this?
Answer
Hint: Both 2 and (p+1)/2 appear as factors in p!.
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