The greatest mathematician of the nineteenth century, and usually ranked with Archimedes and Isaac Newton as one of the three greatest mathematicians of ail time, was Carl Friedrich Gauss (1777-1855). Gauss was one of those remarkable infant prodigies who appear from time to time. They tell of him the incredible story that at the age oh three he detected an error in his father’s bookkeeping. We have already, in Section 5-4, considered Gauss’s surprising contribution to the theory of the Euclidean construction of regular polygons, and, in Section 5-7, his anticipation of the discovery of non- Euclidean geometry. In his doctoral dissertation, written at the age of 20, he gave the first wholly satisfactory proof of the fundamental theorem of algebra (that a polynomial equation with complex coefficients and of degree n has at least one complex root). Unsuccessful attempts to prove this theorem had been made by Newton, Euler, d’Alembert, and Lagrange. Gauss’s greatest single publication is his Disquisitiones arithmeticae, a work of fundamental importance in the modern theory of numbers. Gauss’s findings on the construction of regular polygons appear in this work, as does his facile notation for congruence, and the first proof of the beautiful quadratic reciprocity law, which says, in terms of the Legendre symbol defined