Quadratic Residues and the Frobenius Coin Problem
Mathematics and Computer Science
An odd prime p has (p-1)/2 quadratic residues mod p, and for relatively prime p and q there are (p-1)(q-1)/2 non-representable Frobenius numbers. We show that there is a relationship between the non-representable Frobenius numbers of p and q and the quadratic residues of p that accounts for the presence of (p-1)/2 in both expressions.
Spivey, M Z. "Quadratic Residues and the Frobenius Coin Problem." Mathematics Magazine. 80.1 (2007): 64-66. Print.