Osculating spaces and diophantine equations (with an appendix by Pietro Corvaja and Umberto Zannier)

This paper deals with some classical problems about the projective geometry of complex algebraic curves. We call textit{locally toric} a projective curve that in a neighbourhood of every point has a local analytical parametrization of type $(t^{a_1},...,t^{a_n})$, with $a_1,..., a_n$ relatively prime positive integers. In this paper we prove that the general tangent line to a locally toric curve in $bP^3$ meets the curve only at the point of tangency. This result extends and simplifies those of the paper cite{kaji} by H.Kaji where the same result is proven for any curve in $bP^3$ such that every branch is smooth. More generally, under mild hypotesis, up to a finite number of anomalous parametrizations $(t^{a_1},...,t^{a_n})$, the general osculating 2-space to a locally toric curve of genus $g<2$ in $bP^4$ does not meet the curve again. The arithmetic part of the proof of this result relies on the Appendix cite{cz:rk} to this paper. By means of the same methods we give some applications and we propose possible further developments.