Landweber relaxation parameter ($0<\tau <2/\Vert A{\Vert }^2$)

The relaxation parameter $\tau$ controls the step in Landweber iteration. Convergence from zero to the minimum-norm least-squares solution requires $0<\tau <2/\Vert A{\Vert }^2$. Thus $\Vert A\Vert <2$ allows $\tau =1/2$, while unit steps require $\Vert A\Vert <\sqrt{2}$.