13. 4 3√a/ 3√√a.

$$\frac{\sqrt[3]{a}}{\sqrt[3]{\sqrt{a}}} = \frac{a^{\frac{1}{3}}}{a^{\frac{1}{6}}} = a^{\frac{1}{3} - \frac{1}{6}} = a^{\frac{2-1}{6}} = a^{\frac{1}{6}}$$

Ответ: $$a^{\frac{1}{6}}$$.