6. Given $A_{i}\left(t_{i}+\frac{1}{t_{i}}, t_{i}-\frac{1}{t_{i}}\right)(i=1,2,3$,
4) are four distinct points on a plane. If these four points are concyclic, then $t_{1} 、 t_{2} 、 t_{3} 、 t_{4}$ should satisfy $\qquad$ .