books.google.com - An Introduction to the Theory of Numbers by G.H. Hardy and E. M. Wright is found on the reading list of virtually all elementary number theory courses and is widely regarded as the primary and classic text in elementary number theory. Developed under the guidance of D.R. Heath-Brown this Sixth Edition...http://books.google.com/books/about/An_Introduction_to_the_Theory_of_Numbers.html?id=rey9wfSaJ9EC&q=interval&utm_source=gb-gplus-shareAn Introduction to the Theory of Numbers