What is the condition for a binomial coefficient C(n,r) to be a perfect square? Also, what are the conditions for the binomial coefficient C(n, n / 2) to be a perfect square?
Bertrand's postulate (proved by Chebyshev in 1852) states that there is prime between x and 2x-2 for x>3. This means that C(n,n/2) = n(n-1)...(n/2+1)/n/2.(n/2-1)...2.1 has a single prime factor bigger than n/2 and less than n-2, and so, after checking a couple of small cases it can't be a perfect square.