The continue fractions of quadratic surds are periodic, according to atheorem by Lagrange. Their periods may have differing types of symmetries. Thiswork relates these types of symmetries to the symmetries of the classes of thecorresponding indefinite quadratic forms. This allows to classify the periodsof quadratic surds and at the same time to find, for an arbitrary indefinitequadratic form, the symmetry type of its class and the number of integerpoints, for that class, contained in each domain of the Poincare' model of thede Sitter world, introduced in Part I. Moreover, we obtain the same informationfor every class of forms representing zero, by the finite continue fractionrelated to a special representative of that class. We will see finally therelation between the reduction procedure for indefinite quadratic forms,defined by the continued fractions, and the classical reduction theory, whichacquires a geometrical description by the results of Part I.
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