Problem:
Find all positive integers such that is a product of two or more consecutive positive integers.
Solution
Solution:
Answer: .
Among each four consecutive integers there is a multiple of . As is not a multiple of , it must be the product of two or three consecutive positive integers.
Case I. If (all letters here and below denote positive integers), then , whence . As is prime, this leads to , . Subtracting these yields , , , which is a solution to the problem.
Case II. If , then
Thus each of and can have only and as prime factors, so the same is true for their GCD. This, combined with the identity yields . Now and the latter number is odd, so , . The former identity implies is even and now by the latter one , while in fact — a contradiction. So, in this case there is no such .
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.