Integers of the form x² + dy² in some quadratic fields

Abstract

Writing an integer as a sum of two squares is an interesting problem. David A. Cox has generalized the problem and study that when a positive integer d is given, which prime integers can be written in the form x² + dy² where x and y are integers In this thesis, we extend the ideas from the field of rational numbers to some quadratic fields and give the criteria to determine whether an algebraic prime in- teger can be represented in the form x² + dy² Furthermore, we study the number of representations of an integer which can be represented in the form x² + dy² in a number field.