Square class
In abstract algebra, a square class of a field is an element of the square class group, the quotient group of the multiplicative group of nonzero elements in the field modulo the square elements of the field. Each square class is a subset of the nonzero elements consisting of the elements of the form xy2 where x is some particular fixed element and y ranges over all nonzero field elements.
For instance, if, the field of real numbers, then is just the group of all nonzero real numbers and is the subgroup of positive numbers. The quotient of these two groups is a group with two elements, corresponding to two cosets: the set of positive numbers and the set of negative numbers. Thus, the real numbers have two square classes, the positive numbers and the negative numbers.
Square classes are frequently studied in relation to the theory of quadratic forms. The reason is that if is an -vector space and is a quadratic form and is an element of such that, then for all, and thus it is sometimes more convenient to talk about the square classes which the quadratic form represents.
Every element of the square class group is an involution. It follows that, if the number of square classes of a field is finite, it must be a power of two.
