  Convolution with an appropriate surface measure on a paraboloid is known to
define a bounded operator T from L^p(R^d) to L^q(R^d) for certain exponents
p,q. By a quasiextremal for the associated inequality, we mean a function f for
which the norm of Tf is at least a constant c times the norm of f. Our main
result characterizes all quasiextremals, with some quantitative control in
terms of c. Several related results are also discussed.
  This is the first in a series of at least four articles about a circle of
questions concerning the inverse problem of deducing information about f from
information about the ratio of the norm of Tf to the norm of f.
