  Error estimates with optimal convergence orders are proved for a stabilized
Lagrange-Galerkin scheme for the Navier-Stokes equations. The scheme is a
combination of Lagrange-Galerkin method and Brezzi-Pitkaranta's stabilization
method. It maintains the advantages of both methods; (i) It is robust for
convection-dominated problems and the system of linear equations to be solved
is symmetric. (ii) Since the P1 finite element is employed for both velocity
and pressure, the number of degrees of freedom is much smaller than that of
other typical elements for the equations, e.g., P2/P1. Therefore, the scheme is
efficient especially for three-dimensional problems. The theoretical
convergence orders are recognized numerically by two- and three-dimensional
computations.
