On density of positive Lyapunov exponents for C¹ symplectic diffeomorphisms

Let $M$ be a 2$d-$dimensional compact connected Riemannian manifold and $\omega$ be a symplectic form on $M$. In this paper, we prove that a symplectic diffeomorphism, with all Lyapunov exponent zero for almost everywhere, can be $C^1$ approximated by one with a positive Lyapunov exponent for a positive-measured subset of $M$. That is, the set \[ \left\{ f\in \mathcal{S}ym^1_{\omega}(M)\,| \begin{array}{ll} &\mbox{The largest Lyapunov exponent }\lambda_1(f,\,x)>0\\ &\mbox{ for a positive measure set } \end{array} \right\} \] is dense in $\mathcal{S}ym^1_{\omega}(M)$. \end{abstract} \end{center}