Weierstrass filtration on Teichm\"uller curves and Lyapunov exponents: Upper bounds

We get an upper bound of the slope of each graded quotient for the Harder-Narasimhan filtration of the Hodge bundle of a Teichm"{u}ller curve. As an application, we show that the sum of Lyapunov exponents of a Teichm"{u}ller curve does not exceed $(g+1)\mathrm{/}2$, with equality reached if and only if the curve lies in the hyperelliptic locus induced from $\mathcal{Q}(2k_1,\mathrm{.}\mathrm{.}\mathrm{.},2k_n,-1^{2g+2})$ or it is a special Teichm"{u}ller curve in $\mathrm{\Omega }{\mathcal{M}}_g(1^{2g-2})$. It also gives an unified interpretation for many known results about the special partial sums of Lyapunov exponents on Teichm"uller curves.