The Trace Ratio Problem

Question

For two m × m positive semidefinite matrices $S_p$ and $S_l$, the trace ratio problem is defined as finding a m × d (d < m) transform matrix W∗ that satisfies $$ W^{*}=\underset{W^{T} W=I}{\arg \max } \frac{\operatorname{Tr}\left[W^{T} S_{p} W\right]}{\operatorname{Tr}\left[W^{T} S_{l} W\right]} $$ and the optimum trace ratio value $$ \lambda^{*}=\max _{W^{T} W=I} \frac{\operatorname{Tr}\left[W^{T} S_{p} W\right]}{\operatorname{Tr}\left[W^{T} S_{l} W\right]} $$ How can I prove that this problem does not have a closed-form solution?