CONTINUED FRACTIONS FOR COMPLEX NUMBERS AND VALUES OF BINARY QUADRATIC FORMS

International audienceWe describe various properties of continued fraction expansions of complex numbers in terms of Gaussian integers. Such numerous distinct ex- pansions are possible for a complex number. They can be arrived at through various algorithms, as also in a more general way than what we call “iteration sequences”. We consider in this broader context the analogues of the Lagrange theorem characterizing quadratic surds, the growth properties of the denomi- nators of the convergents, and the overall relation between sequences satisfying certain conditions, in terms of non-occurrence of certain finite blocks, and the sequences involved in continued fraction expansions. The results are also ap- plied to describe a class of binary quadratic forms with complex coefficients whose values over the set of pairs of Gaussian integers form a dense set of complex numbers