  Most engineering models contain several parameters, and the map from input
parameters to model output can be viewed as a multivariate function. An active
subspace is a low-dimensional subspace of the space of inputs that explains the
majority of variability in the function. Here we describe a quick check for a
dominant one-dimensional active subspace based on computing a linear
approximation of the function. The visualization tool presented here is closely
related to regression graphics, though we avoid the statistical interpretation
of the model. This document will be part of a larger review paper on active
subspace methods.
