  We present an offline/online computational procedure for computing the dual
norm of parameterized linear functionals.
  The key elements of the approach are
  (i) an empirical test space for the manifold of Riesz elements associated
with the parameterized functional, and
  (ii) an empirical quadrature procedure to efficiently deal with
parametrically non-affine terms. We present a number of theoretical results to
identify the different sources of error and to motivate the technique. Finally,
we show the effectiveness of our approach to reduce both offline and online
costs associated with the computation of the time-averaged residual indicator
proposed in [Fick, Maday, Patera, Taddei, Journal of Computational Physics,
2018 (accepted)].
