@article{Bam-Eng-Hal-Jol-1988,
    author={Alain Bamberger and Bjorn Engquist and Laurence Halpern and 
           Patrick Joly },
    title={Paraxial Wave Equation Approximations in heterogenous media },
    journal={SIAM Journal on Applied Maths },
    year={1988 },
    volume={48, 1 },
    pages={129--154},
}