11. If the function $y=\lg \left(x^{2}-a x+a\right)$ has a range of $\mathbf{R}$, then the range of real number $a$ is $\qquad$.
11. $a \leqslant 0$ or $a \geqslant 4$. From the problem, $x^{2}-a x+a$ must take all values greater than 0, then $\Delta=a^{2}-4 a \geqslant 0$.