In number theory, quadratic integers are a generalization of the usual integers to quadratic fields. Quadratic integers are algebraic integers of degree two, that is, solutions of equations of the form x2 + bx + c = 0 with b and c (usual) integers. When algebraic integers are considered, the usual integers are often called rational integers. Common examples of quadratic integers are the square roots of rational integers, such as , and the complex number i = , which generates the Gaussian integers. Another common example is the non-real cubic root of unity −1 + /2, which generates the Eisenstein integers. Quadratic integers occur in the solutions of many Diophantine equations, such as Pell's equations, and other questions related to integral quadratic forms. The study of rings of quadratic integers is basic for many questions of algebraic number theory. Medieval Indian mathematicians had already discovered a multiplication of quadratic integers of the same D, which allowed them to solve some cases of Pell's equation. The characterization given in of the quadratic integers was first given by Richard Dedekind in 1871. A quadratic integer is an algebraic integer of degree two. More explicitly, it is a complex number , which solves an equation of the form x2 + bx + c = 0, with b and c integers. Each quadratic integer that is not an integer is not rational—namely, it's a real irrational number if b2 − 4c > 0 and non-real if b2 − 4c < 0—and lies in a uniquely determined quadratic field , the extension of generated by the square root of the unique square-free integer D that satisfies b2 − 4c = De2 for some integer e. If D is positive, the quadratic integer is real. If D < 0, it is imaginary (that is, complex and non-real). The quadratic integers (including the ordinary integers) that belong to a quadratic field form an integral domain called the ring of integers of Although the quadratic integers belonging to a given quadratic field form a ring, the set of all quadratic integers is not a ring because it is not closed under addition or multiplication.