  The magnetorotational instability (MRI) triggers turbulence and enables
outward transport of angular momentum in hydrodynamically stable accretion
discs. By using the WKB approximation and methods of singular function theory,
we resolve two different paradoxes of MRI that appear in the limits of infinite
and vanishing magnetic Prandtl number. For the latter case we derive a new
strict limit of the critical Rossby number. This new limit of ${\rm
Ro_c}=-0.802$, which appears for a finite Lundquist number of ${\rm Lu}=0.618$,
extends the formerly known inductionless Liu limit of ${\rm Ro_c}=-0.828$ valid
at ${\rm Lu}=0$.
