  For Gagen-Poiseuille flow, we show that exponential instability (to extremely
small, axially symmetric disturbances represented by Galerkin's approximation)
is possible only if there exists bi-periodic variability of the disturbances
along the pipe axis when the threshold Reynolds number depends on the ratio of
two longitudinal periods. Absolute minimum (for) is obtained that corresponds
to the observed conditions of transition from the laminar resistance law to the
turbulent one and Tollmien-Schlichting waves exciting in the boundary layer.
