ABSTRACT
A great many things about the number nine and its relationships with the other numbers can be expressed by the equals sign; but there is one property of nine, known since antiquity and both interesting and useful, that cannot be so expressed. This is the fact that divided into any power of ten, nine always leaves a remainder of one. When, at the beginning of the nineteenth century, a notation very like that of the equals sign was at last invented to express this and other similar relationships, all of the numbers took on what might well be called a mathematical “new look.” No single invention in the theory of numbers ever posed so many new and interesting questions. In the history of the number nine lies the seed for this sudden flowering.
