We discuss the role of auxiliary functions in the development of
transcendental number theory.
After a short introduction to Pillai's work on Diophantine questions, we
quote some later developments and we discuss related open problems.
Is it possible to distinguish algebraic from transcendental real numbers by
considering the $b$-ary expansion in some base $b\ge2$? In 1950, \'E. Borel
suggested that the answer is no and that for any real irrational algebraic
number $x$ and for any base $g\ge2$, the $g$-ary expansion of $x$ should
satisfy some of the laws that are shared by almost all numbers.
A basic question of Diophantine approximation, which is the first issue we
discuss, is to investigate the rational approximations to a single real number.
Next, we consider the algebraic or polynomial approximations to a single
complex number, as well as the simultaneous approximation of powers of a real
number by rational numbers with the same denominator. Finally we study
generalisations of these questions to higher dimensions. Several recent
advances have been made by B. Adamczewski, Y. Bugeaud, S. Fischler, M. Laurent,
T. Rivoal, D. Roy and W.M. Schmidt, among others.