We study the convergence of
a formal power series of two variables if its restrictions on curves belonging to a
certain family are convergent. Also analyticity of a given C∞ function f is proved
when the restriction of f on analytic curves belonging to some family is analytic. Our
results generalize two known statements: a theorem of P. Lelong and the
Bochnak–Siciak theorem. The questions we study can be regarded as problems of
Osgood–Hartogs type.