  Young's convolution inequality provides an upper bound for the convolution of
functions in terms of $L^p$ norms. It is known that for certain groups,
including Heisenberg groups, the optimal constant in this inequality is equal
to that for Euclidean space of the same topological dimension, yet no
extremizing functions exist. For Heisenberg groups we characterize ordered
triples of functions that nearly extremize the inequality.
  The analysis relies on a characterization of approximate solutions of a
certain class of functional equations. A result of this type is developed for a
class of such equations.
