Abstract. We prove that, asymptotically, in the set of squarefree integers d, not divisible by primes congruent to 3 mod 4, the period of the expansion of √ d in continued fractions is more frequently odd than even.
Statement of the resultsThe subject of the expansion of the real numbers in simple continued fractions remains a very opaque domain in the theory of numbers. One of the very few achievements of this theory is the following famous theorem due to Lagrange (see [22, Theorem 3 p. 317], for instance). With the classical notations, we have the equalityIn that formula, s = s(d) is the period of the expansion, and we have the equality a t = a s−t , for any 1 ≤ t ≤ s − 1 and also the inequality s < 2d. Hence Theorem A defines an application s : d → s(d), from the set of non square integers to N * . The image of s is equal to N * (see [22, Theorem 6 p.325]) and more precisely for every positive integer s 0 , the equation s(d) = s 0 has infinitely many solutions in d.Recall that if the real number α has an ultimately periodic expansion in continued fractions, then α is an algebraic number of degree 2 (see [22, p.328]
for instance).The application s is very mysterious at many points of view. Here we shall be concerned by the frequency of the odd values of this function. We recall a very useful link between the parity of s(d) and the associated negative Pell equation.