R e T TR : 8 i ’z 8 INTRODUCTION instances to lay emphasis upon the fact that the various branches are in fact closely allied. Number theory is the branch of mathematics which, be- sides being the father of all mathematics, is also that nearest to a non-technical public. This does not mean that the sub- ject is necessarily of an elementary nature. Many of its more advanced problems involve the use of extremely complicated processes of logic, whilst some celebrated problems are still no nearer to any solution whatever. Nevertheless, the greater part of this book is confined to the less abstruse aspects of the subject, and an attempt has been made to render all points under consideration readily intelligible. Where proofs are given or where algebraical notation is employed in any other connexion, the notation has been kept to as simple a form as possible. In ancient times the philosophers ascribed mystical powers to individual numbers, because of the apparently magical quality observable in their construction and relationships. Although these beliefs may have been swept away over the centuries, the relationships upon which they were based still exist and they well repay a little investigation.