  A commonly employed measure of the signal amplification properties of an
input/output system is its induced L2 norm, sometimes also known as "H
infinity" gain. In general, however, it is extremely difficult to compute the
numerical value for this norm, or even to check that it is finite, unless the
system being studied is linear. This paper describes a class of systems for
which it is possible to reduce this computation to that of finding the norm of
an associated linear system. In contrast to linearization approaches, a precise
value, not an estimate, is obtained for the full nonlinear model. The class of
systems that we study arose from the modeling of certain biological
intracellular signaling cascades, but the results should be of wider
applicability.
