  Flow past a circular cylinder is investigated in the subcritical regime,
below the onset of Benard-von Karman vortex shedding at Re_c ~ 47. The
transient response of infinitesimal perturbations is computed. The domain
requirements for obtaining converged results is discussed at length. It is
shown that energy amplification occurs as low as Re=2.2. Throughout much of the
subcritical regime the maximum energy amplification increases approximately
exponentially in the square of Re reaching 6800 at Re_c$. The spatiotemporal
structure of the optimal transient dynamics is shown to be transitory
Benard-von Karman vortex streets. At Re ~ 42 the long-time structure switches
from exponentially increasing downstream to exponentially decaying downstream.
Three-dimensional computations show that two-dimensional structures dominate
the energy growth except at short times.
