  Let f be a G-function (in the sense of Siegel), and x be an algebraic number;
assume that the value f(x) is a real number. As a special case of a more
general result, we show that f(x) can be written as g(1), where g is a
G-function with rational coefficients and arbitrarily large radius of
convergence. As an application, we prove that quotients of such values are
exactly the numbers which can be written as limits of sequences a(n)/b(n),
where the generating series of both sequences are G-functions with rational
coefficients. This result provides a general setting for irrationality proofs
in the style of Apery for zeta(3), and gives answers to questions asked by T.
Rivoal in [Approximations rationnelles des valeurs de la fonction Gamma aux
rationnels : le cas des puissances, Acta Arith. 142 (2010), no. 4, 347-365].
