Notes on Theory of Quadratic Residues
  The Law of Quadratic Reciprocity was conjectured by Euler and Legendre who both found an incomplete proof. Gauss called this law "Theorema Fundamentale", and he was the first who gave a complete proof, he also highlighted the equivalence of his formulation with those of Euler and Legrendre. Hereby notes gives a overview of the Theory of Quadratic Residues using a classical approach with some application to Diophantine Equations, such as Two Square Theorem and Pythagorean Quadruplets. 
Roberto Volpe