star of David

As originally stated by Gould, GCD{(n - 1 k), (n k - 1), (n + 1 k + 1)} = GCD{(n - 1 k - 1), (n k + 1), (n + 1 k)}, where GCD is the greatest common divisor and (n k) is a binomial coefficient. This was subsequently extended by D.

binomial coefficient | binomial sums | Christmas stocking theorem | Pascal's triangle