Law relating (p/q) and (q/p) for odd primes p, q. (p/q)(q/p) = (−1)^((p−1)(q−1)/4). Used to compute Legendre symbols efficiently.. The key formula is (p/q)(q/p) = (−1)^((p−1)(q−1)/4). This concept is typically introduced in College. Understanding this concept builds a strong foundation for more advanced mathematics.