  The mass matrix for Gauss-Lobatto grid points is usually approximated by
Gauss-Lobatto quadrature because this leads to a diagonal matrix that is easy
to invert. The exact mass matrix and its inverse are full. We show that the
exact mass matrix \emph{and} its inverse differ from the approximate diagonal
ones by a simple rank-1 update (outer product). They can thus be applied to an
arbitrary vector in $O(N)$ operations instead of $O(N^2)$.
