6. If $[x]$ denotes the greatest integer not exceeding the real number $x$, then the solution set of the equation $[\cot x]=2 \cos ^{2} x$ is $\qquad$
$\qquad$
6. $\left\{x \left\lvert\, x=k \pi+\frac{\pi}{2}\right.\right.$ or $\left.x=l \pi+\frac{\pi}{4}, k 、 l \in \mathbf{Z}\right\}$